Many people seem to be blinded and think a rider who is an elite TTer can not be a climber. And while that may be true for many TT'ers, it obviously is a completely different story when a rider is small and weighs 60kg. It comes down to watts and weight, and the ratio in his case is quite positive. Another thing in Evenepoel's case is that he's a regular diesel with a terrible acceleration, so he doesn't climb like a small lightweight. He climbs like he TTs. And finally some people think that because he hasn't climbed multiple HC climbs for days on end in a GT, that chances are that he won't be able to do that consistently. Which could be true, since we haven't seen it yet, but then you could ask yourself the question if Sosa and Kuss are good climbers. If you're referring to the comment of Roundabout, that was indeed about his aero build being an extra advantage on less steep climbs. Which doesn't mean he could not be great at steep climbs as well.
I think most of it is wishful thinking that he has at least one big weakness that can be exploited. It's been 3 years that people have been telling us what he can't do, because reasons. I think in many cases it's because those reasons are fear. What if he turned out to be what DNP-old said he could turn out to be, 3 years ago, when he made this topic (and i'm not talking about the title). So far there are many more plausible reasons to assume he can be a great climber, than the contrary. But he hasn't done it, so we "don't know".