Awesome find, unbelievable!
I will go with legit, given on how she looks proportional to the rim and the heels maximum elevation. Vid is bad, the rim could be a couple of inches shorter and maybe she is a couple of inches taller than the 5'11' registered, but overall i would call what i see a legit 10' dunk, respect!
Agree about the too much hype about Griner dunking, no disrespect at all (on the contrary ) but she does have a 7'4'' wingspan and a 9'2'' standing reach. She can dunk with ~15'' vert.