Sami Zayn and El Generico are not the same person.
Sure, they share a moveset, physique, facial hair even (and face, for that matter, or so it's rumoured). What's more, they've never been seen in the same room. But that proves precisely nothing. Hitler was never seen in the same room as Father Christmas: doesn't mean they're one man operating under different aliases (though what a swerve that'd be!).
Not to say Zayn is totally independent of Tijuana's finest. Before hanging his boots up in 2013 (to help save orphans in his homeland, naturally), Generico trained a certain Canadian protégé in his exact image. That explains the similarities, then. Case closed (sadly, Generico later passed away in his pursuit of philanthropy).
This might also explain the image which adorns this article, in which Zayn, confronted with a supposed spectre of his past in the audience some years ago, symbolically turned his back on the masked fan. This wasn't just a repudiation of his fan-favourite history, but a complete and very public rejection of it - a way of putting over his new role as a heel, in other words.
This was the subtlest example of a wrestler turning their back on an old gimmick. Others have been quite a bit more direct.