That could be true of literally any encryption you did not develop yourself and any os or hardware in the same lieu. Apple's implementation is legally binding and third-party verified. That's as good as you're gonna get with any operating system you don't entirely vet yourself, and no a third party linux distribution is not a os you manage yourself as governments local to the distrubuting party/management organization can require backdoors in even those.

In the end you are trusting other humans using their tools to do things you hope you understand but generally fully do not, so you must engage in a modicum of trust. This is just the great abstraction problem in any compute environment. Generally we base this on past or prior actions in combination with behaviors we can monitor or prove. Or should we talk about how pretty much every modern encryption scheme was minted by the NSA?... Just saying... Right that doesn't mean anything and I say that knowing I've got the loaded bullet of apple being the only corporate entity I'm familiar with to risk a public war with the US government by hosting this page despite gag orders while also refusing to implement said backdoors. https://www.apple.com/customer-letter/

In the end, your device will never be truly open source hardware + software, whether its Apple or not, so you will have to trust someone at some point if you'd like to continue using electronic devices.