> He refused to listen to my claim that everyone has a lot of stuff in math they just do by the rules they were taught and only realize later in life that there's an underlying concept.

You're probably misinterpreting "refusal" as "unable". After a lifetime of being told "if you would just apply yourself...", you start to get a distorted impression on what the average experience is. Obviously, he's just coming to understand something that everyone else had appreciated since they first learned it, because they applied themselves.

Maybe I'm projecting -- I'm in a similar spot, but I've been diagnosed since high school, and it wasn't exactly a shocker then.

What you're describing really sounds like the secondary depression[0] that usually comes with adhd. That is, he's probably is being handicapped by the disorder -- it can be a real slog to read math, and very hard to practice it, even when it's interesting. But it sounds like you were coming face to face with depression caused by a lifetime of not feeling good enough. Frustration that you can't apply yourself harder. Being gaslit into believing that you weren't trying.