Now, this of course assumes that engineering managers are not solely doing employee management but also are serving some "tech lead" type roles. I.e. even if an engineering manager isn't coding I think it's essential that (a) a considerable portion of their time is spent doing code reviews, (b) they should be working closely with product managers to define stories and requirements before the dev team starts implementing, etc.
That is, having line engineering managers solely do "people management" is universally a bad idea in my opinion. They should be getting their "hands dirty" in the tech aspects as well in order to be effective.
I doubt though that Musk can figure out the actual workhorse in a short time. The posers are very good at self promotion.
Weeding out those who change pronouns in documentation and agitate on Slack would be a reliable start. From there, close reading of PRs, their usefulness and code quality is required. LOC is not an indicator.
As for managers, most of them are useless.