Nope, Rayquaza Mega Evolves. The meteorite you get at Mt. Chimney is what enables it. Once it consumes it, it has enough meteorite power internalized that it, together with knowing the move Dragon Ascent, can mega evolve without an outside power source. They even spell it out for you in the Delta Episode. The Mega Stones are all meteorites.
Oh I didn't know that, I must have been reading false information on the internet.....
Any Rayquaza can Mega Evolve if you have it learn Dragon Ascent via tutor.
Thank you for this information, this is what i needed!