That's not really how any of this works. The formally verified kernel of the operating system gives you some assurance that your primitives are reliable (unlike on, say, Linux, where you're always a kernel reference tracking bug away from an LPE). But the "application" code you build on top of L4 doesn't "inherit" that formal verification; it's just code, with code bugs. If you formally verify your own code, it's verified, but otherwise you still own your own bugs.

For what it's worth, I'm not sure Apple publishes which variant of L4 it runs on, and there are many of them. The whole formal verification conversation here might be moot.