Nobody is expecting him to reach De Gea's peak level of shot-stopping. He was literally one of the best ever in that regard, probably the best I've seen in the roughly 27 years I've been watching the sport. Onana's pure shot-stopping will not reach that level, it's not even debatable.
But De Gea himself hasn't been remotely close to that level for five years now (bar a brief period in the first half of 21/22). All decent keepers make the occasional amazing save, but the best will do it more often and also won't concede many that they shouldn't. That doesn't describe De Gea anymore.
I'm not saying Onana will definitely be as good or better at shot-stopping as current De Gea, but it's also not a definite that he won't be. We just have to watch to find out, and compare how he goes with the De Gea of the last five years. Unfortunately what is very obvious is that a lot of people are going to pretend that De Gea is still at (or at least close to) his 2012-18 level and compare Onana to that version. 'De Gea would have saved that' is going to be said a lot over the next year or two by people who ignore that current De Gea also wouldn't have saved it. Obviously there is a chance that Onana will be a fairly big drop even from current De Gea but I'd say that's more unlikely than him being roughly the same level.