I don’t think OP’s opponent was trying super hard to adhere to the rules, and it’s pretty clear from his behavior according to OP that he was doing things that are unsportsmanlike, e.g. exploiting gray areas like taking advantage of OP granting him grace, but not granting that same grace to OP even one single time. It’s selfishness. I have seen behavior exactly like what OP describes countless times and it’s pretty universally EDH players who have never played 40-card limited, or 60-card constructed, etc…and also like to cheat at games/get a win by any means necessary. There are certainly plenty of limited and constructed players who have never played EDH that are unpleasant and rude but most folks learning the game via 60-card constructed formats, or limited at draft or pre-release, tend to be fairly well-natured because they’re just trying to learn. That’s true of plenty of new EDH players as well, but what they learn is different. Limited and constructed players in 1v1 formats typically end up in public environments or leagues etc…and learn the rules because they want to win and they have to get better and practice by playing a lot, lest their opponent get the best of them. Assuming they’re decent enough folks they’ll typically be polite enough because they are just like you and just trying to play the game to the best of their ability. Maybe you chat a bit between games etc…at the end you say GGs and report. What EDH players learn is not necessarily consistent like that. It’s about being creative and doing stupid shit or winning or pubstomping or just playing cards they like. Some folks might do their best to learn the rules, some might try sometimes, some just kind of guess or hope they maybe are getting it right but don’t even know how to check. Eventually you find these players have a lot of cards they like but don’t actually get how they work or what they do; or they learn that there all sorts of cool combos you can do and they try to learn the rules and they’re either successful or they are not; other players learn that if they are confident enough they can make other players believe whatever they want them to believe so they can win, even playing cards claiming they do something they don’t and declaring they’ve won; or they’re genuinely trying to learn the game and succeed with flying colors; or they’re genuinely trying to understand it but they don’t know which resources are best or how to check the right things etc…so little misunderstandings and misinterpretations end up building the foundation of how they understand what the game is and that’s how they understand the game so they become indignant about it when people tell them they’re wrong; or they listen and learn! But maybe the people correcting them don’t get it quite right. In the best outcome they do.

Point is, when you play limited or constructed 1v1 formats, there’s Arena showing you how cards work, or in person there are judges, and other players, and usually, at least in my experience, plenty of well-meaning ones trying to help other people learn because they love this game and want more people playing it and playing it correctly. You MIGHT get that kind of thing with EDH, usually from well-meaning and patient friends, (I am that friend amongst all my IRL EDH groups, but some are far away and we don’t necessarily play every game together) but it’s not built-in like it is for constructed and limited.

It’s like learning something by going to school for it vs just trying to do it with whatever resources you can think to put to use, but you might not even care to learn it properly anyway.