If that was never confirmed, it's just speculation.
A Mew made a cameo appearance in the ending credits in ''[[PK24|Meloetta's Moonlight Serenade]]''.
A Mew made its physical main series debut in ''[[SM042|Alola, Kanto!]]'', where it was seen flying around near [[Professor Oak's Laboratory]]. This is the latest main series physical debut of any of the original 151 Pokémon
. It was also implied that [[Rotom Pokédex (anime)|Rotom]] attempted to hunt this Pokémon down to catalogue the Kanto Pokedex during the field trip, but could not find it by the time it returned to Professor Oak's lab.
[[File:Mew PO.png|thumb|250px|Mew in [[Pokémon Origins]]]]