There's a difference between "incapable of understanding" and "doesn't have enough background information to understand." Are there people who can't understand certain tech concepts? Absolutely. But there are a lot more people who just miss the first rung on the ladder, and can't make it to the top. They can understand when they get the explanation from the ground up, but until then, they're stuck.

I see it happen a lot when tech people try to explain something that is brand new to the listener, because when you are already able to understand something at a high level, you forget to mention the first several rungs. It's usually a great explanation, it's just not an explanation the person on the ground can use.

...also, I don't think it's failure to understand login when every instance asks for a separate login if you don't navigate there through your own instance. It's a misunderstanding that results from experiencing the fediverse without understanding how it works, not a failure to grasp an abstract concept.