The joke is that they know it is an egg. The user, somewhat like myself, is clearly trans, and “egg” means something important to the trans community. The extra question marks and the “:3” set the rest of the humorous/teasing tone. The post on here is itself the same kind of subtle meme, with the post text just saying, quite plainly: egg.
I imagine this wouldn’t be intuitive or amusing unless you’re really in the loop.
On the whole site, or just that channel? Because that may be due to channel settings which allow channel owners to hide comments containing certain terms. See “Blocked Words” on:
https://support.google.com/youtube/answer/9483359?hl=en#zippy=%2Cblocked-words
Not that YouTube has a great track record, but forbidding "cis’ would be pretty drastic. For better or worse, many channels choose to filter terms like this to keep their comments sections free of controversy.
The same goes for comments about shadow banning: channels filter terms relating to this so that controversial videos (which many Onion videos will be, especially for those that like eating onions) don’t end up with a comments section that’s nothing but complaints about being filtered, or at least so that the comments section looks the way they want it to. I’ve seen various YouTubers talk about this over the years.