If someone wanted to trick HN users into trusting a phoney key, one way to do that would be to post the phoney fingerprint on HN claiming it to be the real one.
I mean, yes, but you'd also have to have a way to actually MITM the person you are targeting via HN comment, before anyone pointed out it was wrong. It'd be much easier to just use the MITM you already have and not raise the suspicion of posting in a comment.