I think there's a meaningful difference there: "thought-terminating cliché" specifically means something that causes those who say it (or accept it) to stop thinking there. That's not just something that's up to interpretation: that's something that has actual variation depending on the people involved.

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.