A radio tuned to FM does not punish the AM signals by ignoring them. It simply cannot receive the differing signal due to its very nature. It can focus only on FM wavelengths.
OK, I’ve read quite a lot of posts quoted on here with people like this talking about “frequencies” and “wavelengths” and such, but I think this might be the first time I’ve seen one of them actually try to make an analogy to radio. And….yeah, that doesn’t quite work, as I could imagine! A radio will receive any signal that reaches its antenna, it’s just a matter of whether it can turn that signal into anything you can actually hear. I guess they’re halfway right though, in that if you have a radio that’s just intended to be an “FM radio” you can only tune it to frequencies allocated for FM broadcasting.
But hey, I can do something weird like listen to some European “middle wave” station broadcasting in AM on my favorite “WebSDR” (a Software-Defined Radio receiver connected to the internet, allowing many listeners to listen and tune it simultaneously) and I can switch to trying to listen to in FM and….yeah, I can still actually hear it, albeit faintly.
So basically, that analogy is completely wrong.