If you're using an iPhone, you already are trusting Apple to not do scummy things. If they wanted to, they could snoop all your traffic with or without Private Relay.
What Private Relay does appear to do is guarantee that no one besides Apple can do scummy things. That's a definite improvement.
Private Relay prevents everyone, including Apple, from snooping your traffic - assuming it behaves as they document it. Given the protocol appears to be designed expressly to shield traffic from spying by Apple and their partners, it seems reasonable to assume that they're being honest.
What Private Relay does appear to do is guarantee that no one besides Apple can do scummy things. That's a definite improvement.