Unfortunately for reasons that remain murky Nvidia still hasn’t put drivers out for Mojave and their absence from the keynote leads me to believe they won’t have drivers for Catalina. Nvidia blames Apple saying they’re not approving drivers. So yes you can slap a 2080ti in your Mac Pro but without drivers it’s useless.

Anybody know what Apple's problem with nVidia is? Are they still feeling burned from the BGE failures several generations back?

This is a professional machine. No one wants to buy it and use hacky work-arounds, they want good drivers. Which aren't an option with nvidia on macos.