Do you have a citation for this? That is not my experience. With iCloud Relay enabled, all outgoing traffic seems to go through the relay.
Private Relay hides your IP address and browsing activity in Safari and protects your unencrypted internet traffic so that no one-including Apple-can see both who you are and what sites you're visiting
They made it a few years ago they all apps had to use encryption. Long before private relay came out. I have wondered if this was why.