Is that true? When did this happen. If It is true then the same can be done for kairir and she level grind in here to catch up to sora dn riku. Sure its total hacks but is Lea is allowed I dont see why she can't.
It happened off-screen during the course of DDD, Yen Sid only mentions it off-hand in the cutscene after you clear Symphony of Sorcery:
DDD said:Mickey: Master Yen Sid. Gosh, do ya think he'll be able to do it?
Yen Sid: It's clear you cannot teach a cat to bark. But Merlin and the three
good fairies are aiding him in a place that's more...temporally flexible. My
hope is that he can at least learn to wield it. He certainly has fire, so
I suppose it depends now on how strongly it burns.
Nomura confirms it in one of the interviews too, but that snippet from in-game dialogue already confirms it anyways.