It doesn't show how it actually is the only or best way to attain the goal.
And since Apple's implimentation is a black box whos internal workings are not under the users control and not auditable by anyone outside of Apple, it's automatically less secure, in the sense that you should trust it less than some other equivalent that is under the users control and publicly auditable, or just some other mechanism entirely if no other such open platform enclave implimentation exists.
Signal or anything else that doesn't use the secure enclave may indeed be, or may not be less secure than something that does use the secure enclave. The simple existence and use of the enclave does not automatically define superior or inferior security. It also possible that anything else might be less secure, but only on iPhones because or limitations Apple imposes on everything on the platform except for themselves, which I don't think should count.
It's also possible to devise a mechnaism that benefits from the enclave without needing to use the enclave directly. All software on the device can rely on trusting the OS to keep one app from reading another app, because they can trust that the OS itsef can only come from Apple and the bootloader woukld refuse to boot anything else, etc.
There are infinite ways to attain any goal. The way Apple designed their secure enclave and os platform is just one way.
