The pythagorean theorem predicts how long the hypotenuse of a right triangle will be, based on it's sides. That's quite easy to test empirically. You take a ruler out and draw a right triangle. It's nice that we have a solid mathematical proof for it. But if the axioms of geometry were somehow thrown into question, no one would question pythagoreans theorem because we know it works and have tested it countless times. It has more evidence than entirely scientific matters like the law of gravity.