The point is that there is no objective measure one can use and the definition at any given time is dependent on the cultural and societal context.
Something that might have been considered pornography 100 years ago might be considered tame today.
Something that might have been considered pornography 100 years ago might be considered tame today.
It's acknowledging that this is a place where rigid, universal definitions, that include everything that is X and exclude everything that is not X, are not possible. That human judgement is necessary to determine what is or isn't X.
This doesn't mean that we have no guidelines whatsoever for how to recognize X. We can list qualities that a robot either must have, or should probably have, and anything that definitely doesn't have them probably isn't a robot, and anything that has enough of them probably is a robot.
Human judgement in itself is not uniform or defined enough for it to be useful, but dispersed and more random than just situational. And yes, universal definitions in general are extremely rare outside of a subset of definitions in hard sciences. The only reasonable thing we can do is agree on a definition in a certain context, ie definitions in a scientific paper or in a conversation. But 'I know it when I see it' or some vague idea of human judgement doesn't even work as a definition within a defined context, and therefore doesn't help us find common ground or make a conversation useful. Which is why I characterize it as a thought-terminating cliché.
In a way, this entire back-and-forth is an example of us 'knowing what a thought-terminating cliché is when we see it' which unsurprisingly, to me, is unhelpful.
A good example is the use of the term AI, and how we use it in CS vs how the general public understand it.
To put it another way, if we posit Person A and Person B, they might both describe "I'll know it when I see it" as a thought-terminating cliché, because they've heard it described as such, and Person A will actually leave it at that, not seeing any reason to discuss the subject further. But Person B does want to continue discussing, and certainly continues thinking about it. Thus, even though they saw the phrase as a thought-terminating cliché, for them, it actually wasn't one.
On the other hand, terms like "robot" and "pornography" really are purely something that's up to interpretation. If I say "I think a light with a motion detector is not a robot", there's no more objective meaning of "robot" that anyone can point to to definitively say I'm wrong.
It is a non-definition.