Per the WWDC talk on it:
https://developer.apple.com/videos/play/wwdc2021/10096/> In iOS 15 and macOS 12, Private Relay will apply to all web browsing in Safari, all DNS name resolution queries, and a small subset of traffic from apps.
> Specifically, this will include all insecure HTTP traffic, such as TCP port 80.
This implies that app traffic won't apply to HTTPS traffic, which supports my assertion, but then later in the video:
> Not all networking done by your app occurs over the public internet, so there are several categories of traffic that are not affected by Private Relay.
> Any connections your app makes over the local network or to private domain names will be unaffected.
> Similarly, if your app provides a network extension to add VPN or app-proxying capabilities, your extension won't use Private Relay and neither will app traffic that uses your extension.
> Traffic that uses a proxy is also exempt.
So this says that HTTPS traffic will be included, which disproves my assertion, and seems more likely to be true.