Yes, it's considered "normal" by some on 24mm x 36mm, but that doesn't make it true. Several posters in the thread you link describe a simple experiment you can perform if you have at least one working eye to disprove it. Here's another one:
I assume you are reading this at a screen of some sort. What's the angle subtended by that screen from your head position? Now look up at the wall/curtain/window just behind your screen, or the room in front of you. It's a wider angle of view, isn't it? Do you still think your eyesight has a constant "angle of view"?
Finally, for most of last century 24mm x 36mm was not the dominant format. That was 2 1/4 inch square. The diagonal of that is 81mm, and the "normal" lens on that was 80mm.
The diagonal rule has nothing to do with eyesight or physics but is simply an empirical rule based on the types of pictures usually taken. Oddly, it seems to hold true regardless of aspect ratio and is so applied by camera manufacturers.