I mean I highly doubt those glasses are a actual lore object but instead just a game cosnetic for the sake of the discussion (not argument) let's say they were, If those glasses actually had that "pyrovision" effect, that would obviously mean that Pyro wears those glasses all the time, but where? If we assume that he maybe wears them under his gas mask then we would be wrong, because we seen what Pyro looks like and he doesn't wear them, so if we say that maybe his gas mask has those lenses, that also doesn't make sense, cause if you look at those both you would see the color and shape of those lenses are different, so that kind of leaves us with the theory that he actually does see the world in that way (plus on the Official TF2 Wiki it states "Pyro appears to be insane and delusional, living in a utopian fantasy world known as Pyroland.")