I think this was one among several reasons I never found Cain very convincing in the guise of Superman (though I thought he was a great Clark). Though to be fair, Christopher Reeve’s voice was not particularly deep either.
I know, but Reeve's voice didn't have the same back-of-the-throat quality as Cain's, so it sounded more middle-of-the road, if you'll pardon the mixed spatial-orientation metaphor. Now, if Cain had had the voice training to move his voice further forward and make it less nasal and reedy, as I eventually learned to do in order to come closer to my father's rich radio-announcer baritone, he could've differentiated Clark and Superman pretty well.
I did not like the idea of referencing it in the movie when I first heard about it — it seems to me ploddingly literal-minded to think the glasses trope needs to be “explained” — but the way that Gunn used it, basically as a throwaway gag, worked just fine for me.
I feel it would be a better gag if it had just been Guy's conspiracy theory. But that's okay, because I can just geek out about the gorgeous Union Terminal atrium around him. Art Deco meets monumental architecture!