That was an April Fool’s post.
The main giveaway is that the Berserk Gene is an item exclusively in generation 2 of Pokémon, as a held item only obtainable when Mewtwo is encountered as a wild encounter (which, cannot happen). It isn't in any games other than Gold, Silver, and Crystal, and even then cannot be obtained. Therefore, it is impossible to obtain it authentically or without glitches in any game to date.
Tl;dr: You’re doing nothing wrong; it just doesn’t exist. Belated April Fools!!
Hope this helps!
