iOS 12 is out. Split View was added in iOS 9 (2015). Gmail, Hangouts, and many other Google apps still don’t support it. YouTube has Split View but not Picture in Picture.
Is there any reason other than Google management “sticking it to Apple users” that can explain these decisions? Three years is a long time. The lack of these now-common features means they have intentionally decided not to add them. To suggest they have been overlooked would be insulting to the development teams behind the apps.