To think, just two weeks ago, no sane person would have made this claim.
Pokemon is weird, man.
And we are talking a Pokemon with more overall bulk (~8% better physically, ~5% worse specially) than Eviolite Chansey if running its common specially defensive spread. And with actually nice offensive stats, a really solid boosting move and the best STAB a physical Ground-type Pokemon could ever wish to have.
(Zen Headbutt modified as to have the same BP as Psychic)
0 Atk Mew Zen Headbutt vs. 4 HP / 252 Def Eviolite Chansey: 141-166 (21.9 - 25.8%) -- 2.6% chance to 4HKO
0 Atk Mew Zen Headbutt vs. 252 HP / 4 Def Zygarde: 82-97 (12.8 - 15.2%) -- possible 7HKO
0 SpA Mew Psychic vs. 4 HP / 252+ SpD Eviolite Chansey: 46-55 (7.1 - 8.5%) -- possibly the worst move ever
0 SpA Mew Psychic vs. 252 HP / 252+ SpD Zygarde: 73-87 (11.4 - 13.6%) -- possible 8HKO
How we underestimated something with that bulk is anyone's guess.