The word "proves" in that maxim means "tests", in much the same way that bulletproof armour once meant that the actual armour in question had withstood a bullet

The word "proves" in that maxim means "tests", in much the same way that bulletproof armour once meant that the actual armour in question had withstood a bullet (and you could see the dent) or that the proof of the pudding is in the eating.