Exactly. If you look at YouTube bitrate for a 1080p video, it's 8mbps, which is 1.67% of the theoretical max data rate of USB2 (480mbps), so there's more than enough bandwidth for CarPlay.
Yeah USB3 and greater is really only needed for high res displays and faster large storage.
given that carplay has, until now, been exclusively implemented by devices that don't support USB3, that's a pretty good suspicion