You just pointed out the difference between a pianist and keyboardist. There's absolutely no way that one could develop muscles required for standard piano literature. That's exactly why heavier keys cause fatigue for those who play keyboards. It takes me less than 10 seconds to figure out a non-pianist performance on a standard acoustic piano, even if they're a skilled keyboardist. Uneven touch, and inconsistent dynamics -amongst other things- give it away.

Same thing with a Pianist playing the keyboard. It just looks and sounds odd. Unless one is trained on both, these two terms are not to be confused.