From my point of view, the comment you’ve linked to makes sense as-is. So, I personally would say it’s fine in that particular case
But: sometimes (hinting at your linked case… but then again, the question is off-topic in itself) starting to write TeX in a comment may just point at the fact that that comment could be a worthy answer… which means that before hitting the “post comment” button, everyone should do a self-check to see if he/she isn’t actually posting an answer as a comment (which I think would be a shame, because in that case it most probably deserves upvotes and maybe even an “accept”… which isn’t possible to do when the answer was posted as a comment).
Aside from that, the only thing that actually has a chance to worry me is the
$$$$ case which inserts two (or more) entirely blank lines. To be honest, I wasn’t aware of that one yet. The problem I see with that one is, that it would add too much whitespace, pushing the page into unnecessary length (thereby destroying a bit of the usual, overall readability). I think that would somewhat mark the line of “pushing it to the limits”.
So, the line between acceptable and abusive would be somewhere between using a bit of TeX to make a comment clearer (like what you’re linked to), and using multiple instances of
$$$$. After all, a simple comment with a bit of
$$LaTeX$$ still looks fine and may even enhance readability of that comment. But it would start to look a bit abusive when using something like
$$$$ $$$$ $$LaTeX$$ $$$$ $$$$.
Providing some visual examples:
Acceptable: TeX centered on it’s own line looks fine.
(Especially when sharing somewhat long formulas.)
Acceptable: TeX within the text looks good too.
(Depending on what’s TeXed, is might even look better/cleaner.)
Abusive: Multiple blank lines don’t really make sense in a comment.
(And there are less obnoxious ways to grab attention.)