Ha! Don't give me too much credit here. I don't have the resources of a Digital Foundry, let alone the technical prowess (I still can't pixel count worth a damn, haha).
The only definitive source I was able to find, and it was completely by accident, is Microsoft. Who'd have thunk it?! It bears mentioning that the Xbox One S can do VRR, same as the X, but I am limiting my findings only for X Enhanced titles.
Oh, nice! I had a little bit of time to fire up Apex Legends for the first time after work yesterday. It definitely doesn't look 2160p. I wouldn't be surprised if it performs the same as Titanfall 2, fluctuating from 1800p to 1440p, as I thought the image looked a little soft (maybe closer to 1440p?). I'll defer to the experts who can pixel count (Read: not me) for the final say, though.