According to this bug it was shipped six months after the bug was created and they didn't seem super familiar with it.

It was public knowledge that Apple had a shipped implementation using this instruction before that patch was merged.

https://news.ycombinator.com/item?id=18163433

I also don't really see any indication that the project maintainers don't know about the instruction.

Looks like an example of JSC not being developed in the open more than anything else.

are you referring to the tweet linked there? That tweet is wrong in a lot of ways, because that instruction gives you a 1-2% boost, not a 30% boost like they claimed.

They're talking about a 30% boost overall on a new release which is an amalgamation of many pieces, including use of this new instruction.

Can you give an example of something the maintainers said that made them seem "not super familiar with it"?