You can tell security is real the same way as lots of other things, reality doesn't give a fuck. Like how you can tell the difference between man's laws (e.g. "The Offside Rule" or "Constitutional Rights") and Mother Nature's laws (e.g. Thermodynamics). Try it, kick the ball even though the rule says you mustn't - if you get lucky the referee doesn't notice and play continues. But if you try to make a system more ordered without expending energy it does not work. Reality doesn't give a fuck.

When I breeze through your login process with the wrong credentials that's because your security was fake, if it was real that would break because it didn't know who I was, so if some bug lets me past login I don't somehow successfully log in as me, I'm logging in as nobody at all which is clearly nonsense.

This is "Make Invalid States Unrepresentable" at scale, and it's difficult to do, but not impossible.