This could be just that in the Japanese versions, you get Mew when you meet the diploma's conditions. And since the game only checks if you have 150 Pokémon captured, it makes sense that obtaining Mew or Pokémon 152 gives you the diploma.
Here is the code that counts the number of Pokémon owned : https://github.com/pret/pokeyellow/blob/793c715ba253e08d2807d04d128765ca305d6b15/scripts/celadonmansion3.asm#L5
Since it counts all set bits in the Owned Pokédex, including #151 and #152 (both for code simplicity and because they are never intended to be used before completion [#151 in JP] / at all [#151 in non-JP & #152 in all]), the game sees you have >= 150 owned and gives you that reward.
So that's just a consequence of glitching the game to get those bits set when you shouldn't, and doesn't hint at any removed feature.
Just put this there in case someone is wondering where it comes from :P