Ultra HD BD trades 25 GB media for 33 GB media (single layer) and 50 GB media for 66 GB media (dual layer). Those two capacities aren’t in the original BD specs, so current drives just weren’t designed for them in mind from a firmware standpoint. (LG’s latest drives may or may not technically support them, but there’s no confirmation, & it’s not an official feature).
The last capacity, 100 GB, is already equivalent to what BDXL offers, but I doubt it will be perfectly compatible.
While the firmware on existing drives could probably be tweaked to recognize the media & read it at the correct speeds, I assume there’s probably some sort of additional licensing cost that’ll prevent that.
Note: none of this says we’ll be able to BURN 33 & 66 GB media, or that such (re)writable media will even exist.