I used to work there - the jane street code review software is awesome, kind of like graphite but it works reliably. You can write a big tree of PRs. PRs get reviewed separately and you can merge them in any order at your leisure, without worrying too much about rebase issues or clobbering review or whatever. I would love to have some open source thing like that that actually works nearly as well. It may exist, but I haven't seen it yet.

And yeah, jane street is a pretty compelling demo that A) NIH syndrome is fine if you're good at writing software and B) it doesn't really matter that much if you use a mature language or some uncommon immature language