But wouldn't mew need a trainer with a z ring and it's crystal to do that I mean I know Mewtwo mega evolved without a mega stone in the last bw movie but that was different
How so? None of the XY movies portrayed Mega Evolution correctly either.
The diamond Diancie gave Bonnie somehow became a Mega Stone, and it wasn't even holding the diamond when it Mega Evolved.
Rayquaza, Latios, and Latias all Mega Evolved without Key Stones or Mega Stones either.
And the Volcanion movie just made up an alternate means of Mega Evolving with the Mega Wave.
Movies do whatever they want.