Ultimately, what can you really verify as a user? Apple is very different from bitcoin, or IPFS or matrix or the like, in that they have an existing reputation. Instead of trying to provide perfect guarantees, they make a real attempt at preserving privacy, and you just have to trust them on that. I personally trust them after having attempted experiments stymied by the privacy team, but i think in general there's reasonable evidence that apple really does try to be privacy-preserving.
For your example, sure it's possible for the server to do something you don't know. But it's the same people on the server doing the aggregation as on the client doing the obfuscation. If they really wanted they could easily e.g. set the seed for the noise to be based on a key which they have on the server, which would be very difficult to detect. I think you just have to trust that apple wouldn't let that happen.