A hard row to hoe
It's not clear to me why developers would use Samsung's API, toaster-fridge compatibility or no, given that Samsung's only got about 25% of the Android smartphone market. By programming for Google Assistant, a developer can address 100% (well, modulo quite a few smartphones that use Android but not Google's goodies). That's not to say that (unlike constrained Siri functionality on iOS) either API is available yet: Samsung says "eventually", and Google invites you to sign up to a mail list for announcements. Who knows who will be first out of the gate? But I'd back Google to win over Samsung, whatever.