Anyway, being a philosopher simply isn't what the guy did and academics are fairly dismissive without consideration anytime religion (and specifically Christian apologetics) gets in the mix.
In fairness, it's a good heuristic. Most Christian apologetics are just less poetic subsets of the Book of Job, but their authors act like they have some new and exciting insight that will prove Christianity 100% for sure this time. It's a total waste of time to engage.
C.S. Lewis's works are the exception, not the rule, since there's actually something there to engage with. Plus, he's just a good author.
It's also really weird in fields like archaeology/anthropology in North American institutions where western religious texts are given no consideration (not that they should) whereas Native American creation myths are given a kind of deference. White guilt is a powerful thing.
I admit I haven't read that one myself; my own take on him as a philosopher stems from reading Mere Christianity and summaries of Surprised by Joy, and not being aware of any references to his work by more recent philosophers that I've been interested in such as Chomsky, Zizek, or Dennett. I got a strong feeling that Lewis' arguments and exposition were guided by something other than logic, though they pretended to be following it. The trilemma is an example of this.
One way to discover that we necessarily have worldviews outside of logic is to look at a statement like this, and realize it appeals to something outside of logic (feelings) to critique how someone else is only pretending to follow it.
But - with his trilemma, Lewis claims "here is a logical argument for believing in the Christian god", and then presents a weak and illogical argument. It's difficult to know what to make of this except that Lewis was somehow blinded to the logical flaws.
That rules out "did not actually exist historically" and "others made up all the God stuff over the years". (I mean, those exists as possibilities in an absolute sense, but not as possibilities that he had to answer, because the people he was answering didn't hold those as options.)
And to me, "a nice guy trying to help people and he thought tactically the mystical claims would allow him to help more people" sounds a lot like "liar". Liar with good intentions, but still liar.
I'd also say that, even accepting the gospels, we have to really take wide views of "liar, lunatic, or Lord" to make it work. Lewis himself further described the "liar" as akin to the "Devil of Hell" - I wouldn't characterize the liar I described as such. And he said the lunatic was akin to "the man who says he is a poached egg", but people are great at compartmentalization and can hold some very silly specific beliefs while still being otherwise rational, sane people. Even "Lord" admits plenty of things we wouldn't recognize as Christianity, e.g. rejecting the trinity or plenty of other heresies.
If you're going to accept Jesus as a good human teacher, but not accept that he actually taught what the gospels say he did, then you have a good teacher but no teachings. And maybe that's a position that a lot of people like, because it leaves them an empty figure into which they can pour whatever teachings they personally favor, but I think it's rather a cop-out. (A steelman version would be: "I think he was a good human teacher, but no reliable record of what he actually taught survives, so we cannot claim that any particular teaching or position has his stamp of approval." But I still think that's weak. If you can't trust the record of what he taught, how can you trust the record that he was a good teacher?)
Accepting the gospels, the liar you describe is still offering people eternal life, at a price - "take up your cross and follow me". (The cross was not just a burden, it was an instrument of execution.) If the eternal life isn't there, it's hard to describe that as a benign or beneficial lie. Note that many of his followers were executed. (Our record of that doesn't depend on the gospels - Tacitus also says this.) You may say that this still falls short of Lewis's description, but it's far from your description. (I'd have to go back and reread Lewis's argument to see how tightly he has boxed people in on the "liar" branch.)
If you don't actually believe in the supernatural stuff it seems like the definition of a benign lie. He thinks having society follow his broader teachings would result in a much better society on net, regardless of some people being killed for following and spreading it. He thinks that you go in the dirt when you die so he says, hey, actually you go up to this great place. This really helps it catch on and spread. Nobody finds out that part's not true because when they die they just go in the dirt.
For example, I could wave a magic wand today and fix climate change, saving millions or billions of lives. But since I did so, some granny slips on a patch of ice next winter and dies from the fall. Without my magic wand, she would have lived.
Now, I don't specifically know who this granny is before I choose to wave my wand, but as a thinking individual I understand that some such granny must exist. In fact, there are probably many grannies who will die as a result. Would you argue that waving my wand is not a positive change because specific people exist who will be harmed?
That is definitely not the point of the trilemma. It simply argues that Christ must be placed into one of three categories, which do not include "generally nice and completely harmless moral teacher". He was clearly claiming to be God, which obviously leads to the trilemma of choices between Liar, Lunatic and Lord.
That doesn't say which one of those three to pick. Just ruling out the other possibilities by considering the claims he made about himself.
I don't know if that's true. Certainly there are biblical scholars who would disagree. It's not mentioned in the first three gospels, for example.
> obviously leads to the trilemma of choices
Again, not obvious. The three choices are logically incomplete. There are other possibilities, such as that Jesus was not a ful-on lunatic: he had a single delusional belief about his own divinity but was otherwise rational.
Your comment led me to this: https://www.lewisiana.nl/marymidgley/
The natural follow-up question -although probably hard to answer - is whether the existing philosophy faculty includes others like Midgley who have a positive view of Lewis, to the extent that it would influence this decision. I would have expected most of the philosophers there to be in the analytic tradition, but happy to be shown wrong.