It does seem like a sensible thing on the face of it, like many other Session Zero discussions. But then a DM needs to worry about enforcing roleplaying standards on top of all their other responsibilities - even if it comes up very rarely, it's still there.
So, I expect you're thinking far too much into this, in terms of how many details and how much cognitive load in enforcement is required between people of generally good intent.
"Hey, Joe, your character there has an Int of 8. How were you thinking of running with that?"
"Well, I figure that's the lower end of average. Basically an Average Joe (yeah, I know, I didn't name myself, get off my case). He can read, and do basic arithmetic, but isn't going to be winning any puzzle-solving championships. While he knows how his own abilities work, he's not going to be a tactical genius. I'm not taking any Int-based skills for him."
"Okay, because I've had some people with Int 8 try to play Sherlock Holmes, and it was really weird. Glad we are on the same page."
The important point here is having the player acknowledge what the stat means up front, rather than allow them to ignore it. And, having had that discussion, you both have expectations (you of his thoughts on approach, him on the idea that stretching up on that might not work out). In all likelihood, you'll never have to refer to it again - having acknowledged things, the player is likely to stick by it. If you do have to visit, you've already established a general reason for it. There'll be no surprise that you think there are limits on what's appropriate for that level of Int. And, you can refer back to the expectations, and dig into why they aren't holding to them.