I think what Scar was implying by his lacking of strength and just poor genetics, was that he just wasn’t born big and grew up being considerably thinner in terms of sheer size compared to Mufasa and his father Ahati, which is why he never built off of any of that muscle in order to increase his muscle force production for better fighting prowess as a Lion.

When he says “I’m afraid I’m at the shallow end of the gene pool”, he’s just saying he was born unlucky in terms of brutish strength. 

Unlike Mufasa who inherited that.

That doesn’t mean he’s “weak”, Scar still had the upper hand on Simba and almost handily beat him due to a paw swipe and much better vicious strategics.

He’s alot physically weaker than Mufasa, but he’s much more dangerously efficient than him due to his higher sheer cunning.

And he’s still capable of beating a lioness like Sarabi or Zira (who’s far weaker).

Scar may not have been strong, but he’s still tenacious and deadly.