As a former metalhead, I think Rush is quite "metal" compared to other prog rock bands I've heard. I never got into Pink Floyd or Yes and that's why I've been ignorant about Rush for so long, because I had no interest to dig deeper to that kind of music.
Some examples of things Rush shares with metal music:
-big, heavy sound
-individualistic attitude, integrity as the highest value
-"art as expression"; not preaching political messages, getting girls, money or such things
That's what I liked about metal and also like about Rush. So to me Rush is "metal" in that positive sense. Because in no other genre you can find those features. But it's still metal as an adjective, not the ultimate definition, so both sides are right in their own way