It's not that daft - but I understand why it seems so. The stat is how many goals the keeper has stopped above and beyond what's expected.
Clearly the keepers facing a lot of shots can do better here, hence Luton, but you can take that out by looking at the percentages.
If you're conceding 30 goals instead of 34 - that's nowhere near as good as conceding 10 instead of 14, despite both keepers preventing 4 goals.
In percentage terms in the PL this year Onana is still second, with only Alisson ahead of him. Alisson, despite making plenty of clangers against City, has consistently been one of the top few shot stoppers in world football, so that's much more impressive company to be in.
Onana was also, by some distance, the best keeper in terms of preventing goals in the Champions League last year. He's arguably the main reason why Inter got anywhere near the final.
He's clearly a bizarre keeper, given how many errors he's made already, but Ten Hag is right, that even with those errors, he's helped United significantly in the PL this season.