As someone else said, it's not a security barrier between the ARM core and the radio hardware bits... It's more a tool for remapping stuff to make system design easier, and as a way of protecting against evil hardware outside the SoC.

I believe it can prevent the ARM core tampering with private radio hardware memory, but not the other way round.

I think Qualcomm XPUs can be used as barriers (pretty much the same way SMMUs are used).

XPUs + hypervisor should be enough, assuming Qualcomm enables them and configures them correctly.