It's not the same commitment. When doing a commitment through a partner, you're doing an expense commitment (let's say, 600k in a year) in ALL your expenses. Well, except for Google Maps API it seems :P. So not tied to an specific product or type of instance, as the typical commitment, but to your whole GCP billing.

From this, you get a wide range of discounts in a bunch of products, not just instances. And I think those discounts go on top of some of the other discounts you regularly have, but I'm not sure and I'd had to check our billing.