That's a consequence of the Von Neumann architecture, not a tenant of it. All a Von Neumann machine is is one where both code and data exist in the same address space and are stored in the same memory pool (plus some other details). Apple's devices still do that, all the restrictions take place in the OS - well above the computer architecture level.

You might as well argue that Linux isn't really a Von Neumann architecture because you can only execute files that have the executable bit set.