as Einstein the only guy in the history of the world that could've discovered general relativity?

Poincare's "relativity principle" and Lorentz's transformation from one "local time" to another predated Einstein. Relativity was already discovered before the "theory of relativity".

Basically Einstein just sexed it up a bit. Like how Turing gets credit for the "universal computer" even though Church's lambda calculus is earlier and more useful - but Turing's Machine was an easier sell to the populace and he was pushed by special groups (the British and the gays), just like Einstein.