According to this article he's one of the best keepers in the Premier League:
https://the18.com/en/soccer-entertainment/epl-goalie-rankings-2020
Not sure exactly how they work things out though, because they talk a lot about the saves a keeper is expected to make, and in the tables (
https://fbref.com/en/comps/9/keepersadv/Premier-League-Stats), he seems to be around the middle for those stats.
I assume they've combined them with other sources, or he's a good all rounder - apart from the obvious that his pass completion is the best, he seems to be in the top third of just about every aspect (e.g. De Gea is good at stopping shots, but awful at collecting crosses).
I guess it boils down to what a keeper is. We focus on the shot stopping v. the way he plays the ball out from the back when we're pressed - but he's also very proactive; strong at collecting crosses, acting as a sweeper, and distributing accurately over long distances.