It’s both I think. The altitude is obviously a big deal but also it’s CONCACAF craziness. I’m not saying England doesn’t have to deal with some interesting atmospheres in some UEFA countries but England playing at Azteca feels like a fish out of water.
on the other hand you’re right that most of their competitive games there are against CONCACAF minnows. And while the US has never won a legitimate competitive match there, they are technically undefeated in their last 4 games there (1 win, 3 draws). So yeah I get it, Azteca mystique is probably bigger than their actual advantage, but England playing there also feels very interesting. We’ll see how it goes, either way it’s one of the more intriguing games I can remember in years.