Oh, I’m most certain there’s “a way” apple can verify the numbers “independently”.
But, unless Google gives Apple the keys to all their back end ad data and revenue processing engine (not going to happen), they’re basically going off of numbers that Google is providing them with. OR going off of numbers that some “independent” third party has ascertained… using Google’s tools.
So, yes, in this and in a lot of tech businesses engagements, it really does amount to “trust us”. As long as the dollars are in an acceptable range, Apple’s not going to quibble over 47 users they’re sure searched from Chippewa County.