I wonder if that makes a difference in practical terms.

If only by its effect on yourself and other people. If you taboo "smarter" and replace it with "more knowledgeable" or "large inferential distance", you do not claim that one can't reach a higher level:

"That person is smarter than you." = Just give up trying to understand, you can't reach that level by any amount of effort.

vs.

"That person is more knowledgeable than you." = Try to reduce the inferential distance by studying hard.

I first encountered humans who couldn't understand basic arithmetic at university...

I believe that to be the case with literally every new math problem I encounter. Until now I have been wrong each time.

Basic arithmetic can be much harder for some people than others because some just do the logic of symbol manipulation while others go deeper by questioning axiomatic approaches. There are many reasons for why people apparently fail to understand something simple, how often can you pinpoint it to be something that can't be overcome?