This is a head-canon, but: I genuinely believe that, in the games' universe, Pokeballs break/disintegrate during failed captures. (which is why we can't retrieve/reuse already spent balls, like they sometimes do on the anime)

Ergo, no matter how strong Machamp grip is, in accordance to game-mechanics, Mewtwo would just break/dissolve the Pokeball from the inside, releasing himself and retaliating with his type-advantage, stopping Red from trying this tactic once again.

…still, it's WAY funnier imagining that this actually works, and no one has been clever enough to try it before.🤭

Done in a standard poke ball too