The thing is that in this example, person A is saying something that is clearly contrafactual. Sea-lioning usually involves things that require a modicum of judgement, which makes it something that can be questioned. In addition, it is all too often a group activity, either as part of coordinated harassment or because people can't be hedgehogged to read a whole thread before replying. So what you get is something like this:
A: X said a bad thing.
B: What do you mean X said a bad thing?
A: X said Y, which is bad because of these reasons.
B: Where is your proof that X said Y?
A: Right here.
B: I don't know if I can trust that source.
A: Well, there's also this and that source where X says something close to Y, only using different phrasing.
B: I don't think Y is as bad as you say it is. And doesn't X have freedom of speech?
A: X certainly has freedom of speech, just as I have the freedom to say that saying Y is bad and that people who say Y should be shamed for it.
That bit is exhausting enough, but then we add:
C: What do you mean X said a bad thing?
... with C, D, E, and F all repeating the stuff B said.