It's completely standard practice for SoCs to have IOMMUs these days. E.g. the Apple M1 has over a dozen coprocessors doing various things and sharing memory, but only the GPU coprocessor has access to OS memory (because it manages GPU page tables; we're working on figuring out exactly what the risks are with that one). Everything else is firewalled off.

Well, most of them anyway. The one in the Librem 5 does not AIUI.