“You write your snide bullshit in a dark room because that's what the angry do these days”
My best take is that you take the general expectation or main thing about a situation -- Then when something is "extra," the magnitude of the expectation or thing is large. But usually there's an unusual spin involved too, only very slightly.
Example: My fiancee's makeup is extra. Makeup is typically accenting beauty and lightly applied, so when it's extra, it's overwhelmingly pretty and heavily applied.
In this context, Hacker News is known for mostly serious discussion and polite chains of building thoughts. The users are extra today by being dramatically serious and having impolite chains of volatile thoughts, or maybe even chains of echochambering.
If her makeup is "over the top" that's a bad thing, she's not dressed appropriately. If her makeup is "extra" it's meant to be encouraging -- it's "over the top" but the "inappropriate" change is welcomed.
"Extra" is sometimes used with that sarcastic element or downward twist (seen by OP) that chips away the welcoming bit. In that context, I think the usage is similar to "over the top".
Thank you for the explanation.
Hopefully more help than ramble. Thank you for coming to my TED talk :)