This is actually not true!
librarian nerd warning signal flashes brightly
US copyright law grants copyright to any fixed work, which it defines as follows:
An original work of authorship is a work that is independently created by a human author and possesses at least some minimal degree of creativity. A work is “fixed” when it is captured (either by or under the authority of an author) in a sufficiently permanent medium such that the work can be perceived, reproduced, or communicated for more than a short time.
In other words, a comment that has been “fixed” by appearing on YouTube and that was written by a human with some amount of creativity (which this one definitely does) is definitely covered by copyright. Something like “lol” would not be, but this one for sure meets a reasonable definition of “creativity.” Or, if you prefer, originality. (source)
There are some things that can’t be copyrighted, which you can see listed here, but this isn’t one of them. (Incidentally one thing that can’t be copyrighted is recipes, which is why most recipe blogs are so dang hard to use. They add all that nonsensical, non-recipe blather in front of the actual recipes so they can protect the blog post under copyright.)
Of course, it’s highly unlikely a random YouTube commentor will sue you for quoting them in a choicescript game. So, really, you probably still aren’t under any real obligation to cite your source (although from a legal standpoint it would be safer to get permission).
But if, for instance, you were to print that quote on a t-shirt and make ten million dollars off it, the YouTube commentor could sue you and probably win–so long as they can prove that the quote is their intellectual property (which it was as soon as they posted it to YouTube, since YouTube’s TOS makes it very clear they do not own the copyright of content on their platform). Basically, they’d just have to have some kind of convincing proof that the quote is their intellectual property, and that you made money off it without their permission.
@RascaldeesV2, this is the commentor’s profile, which lists their name. Since the comment is pretty recent (3 weeks ago), it doesn’t seem like it would be too hard to reach out by replying to the comment where you saw it and asking for permission to quote it in a game you’re writing, with credit to whatever name the commentor prefers. (Or they might just say you don’t need to credit them, but then you’ve done your due diligence!)
Honestly, even if it’s not legally required, crediting people is polite and not that hard to do most of the time. But obviously I’m a librarian, so it stands to reason I would have, uh, opinions about things like accurate citations. 
(@void_mermaid’s suggestion is pretty funny, too, though!)
((PS note that “by a human author” thing? That’s why anything generated by AI is probably going to end up not covered by copyright, although I think this is still an active discussion.))
