He may be slightly below Kimbrel production-wise, the difference is that Diaz basically makes the league minimum. As an asset, he’s about as valuable as a reliever can get.
I really think the Mariners should have dealt with Cano separately from Diaz to maximize the prospect return. Teams are very hesitant to give up blue chippers nowadays, but rare assets like Diaz can land a centerpiece like that and at the beginning stages of a rebuild that should be the priority.