While the ring floor dampens impacts a bit, it does not prevent your brain from crashing into the walls of your skull.
I'm also pretty sure that all professional fighters have CTE which accumulates over time and eventually a blow normally not enough to do that kind of damage will be enough. The elbow blow itself didn't seem to have been that bad, considering that it happened in the last round when fighters have spent most of their strength and speed, but the impact on the ground didn't look good.