No, the problem really is the prefetch itself when there are undesirable side-effects if the bus sees that memory read.

Well yes. That is why the "prefetch" is a problem.

But the original question was asking why disabling data prefetching to a memory region didn't automatically disable instruction prefetching at the same time.

And the answer is that speculative execution is a completely different mechanism that I'm not even sure can be disabled, at least not per memory region.