garble actually sounds like an excellent utility to add some protection around things like keys/secrets in a binary. Is there anything like this for Swift binaries?
As such, you should never use them to protect data that needs to stay secret indefinitely (or for a long time), such as keys.
Apps and websites get copied all the time. Somebody throws up a duplicate with ads and steals your traffic and search rankings and customers and whatever.
Adding code to prevent your product from working when it's not on the right app/domain, and obfuscating your code to obfuscate those checks, can be sadly necessary. It doesn't need to defeat a determined attacker, but just be hard enough they'll spend their time cloning something else instead.
I speak from experience...
Unless you want your app to be used anonymously, but then why have secrets?
So the app used a digital signature / request signing with a key that was obfuscated and embedded in the binary. With anonymous users I don't know how else you could avoid use of the private API.