So, it's basically an honor system. You cannot detect JIT, because there aren't "certain instructions" that aren't allowed - it's just certain registers that programs shouldn't access (but access patterns can be changed in branching code to ensure Apple won't catch it in their sandboxes).
Besides, even if certain instructions are not allowed, a program can modify itself, it's hard to detect if a program modifies itself without executing the program under specific conditions.
You're missing the point, JIT not allowed means programs may not modify themselves. They're in read+execute only memory and cannot allocate writable+executable memory.
So, it's basically an honor system. You cannot detect JIT, because there aren't "certain instructions" that aren't allowed - it's just certain registers that programs shouldn't access (but access patterns can be changed in branching code to ensure Apple won't catch it in their sandboxes).
Besides, even if certain instructions are not allowed, a program can modify itself, it's hard to detect if a program modifies itself without executing the program under specific conditions.