I wouldn't agree either, but I think there's some kind of logic: At a certain point familiarity can be a detriment to learning if it leads to you adding invalid assumptions to your mental model because everything else is so familiar. If everything is unfamiliar however you're less likely to start making assumptions.
As for how true of effective this is, I don't know. Anecdotally however I had less problems learning entirely different keyboard layouts for example as opposed to layouts that are just slightly different.
Technically you can do everything through email, because everything online can be represented as text. Doesn't mean you should.
PRs also aren't just a simple back and forth anymore: Tagging, Assignees, inline reviews, CI with checks, progress tracking, and yes, reactions. Sure, you can kinda hack all of that into a mailing list but at that point it's becoming really clunky and abuses email even more for something it was never meant to handle. Having a purpose-built interface for that is just so much nicer.