As far as I know it's the same principle as with "premium accounts". For example Linkedin; you can have premium features on iOS even if you bought premium on Linkedin website or somewhere else. The same applies with the wallet. It's fine to send sats to your Primal wallet as long as there is an option to buy them through Apple IAP *and* there is no mentioning in the app about being able to do it outside. Like in Spotify for example.
That seems like the closest approximation. But still there is something different about the fungibility of sats versus a feature that can only be bought once.
In any case, kudos for getting this in place and maybe we’ll see Apple formalize the legitimacy of this use-case in the rules so that more apps can confidently go down this path.