Technically any creative work you author is copyrighted, so your comment was copyrighted anyway.

Whether or not large tech companies will punish people for infringing upon your copyright is yet to be determined.

I don't know about US lawy but in German law there is a requirement on some originality. ("Schöpfungshöhe") The barrier is low, but a 1 certainly doesn't receive the protection.

Yep. A HN comment a couple of sentences long, OTOH, is probably just on the threshold where whether it counts as a protected work depends on just how “unique” the comment is.

As we are on HN, not reddit, thus comments are more than "yes" or "lolz" they are most likely protected.

However one implicitly gives a license to YCombinator - at least to publish it. And what other rights can be upheld would be interesting.