Of course, otherwise how would they support passthrough legacy HALs for devices upgrading to Oreo as described on the documentation?

Interprocess RPC is only for Oreo and newer.

The current HAL works via dynamic libraries that get loaded into your process and talk to kernel space with a platform specific API. The passthrough support is just opening both sides in the same process, loading those legacy libraries, and wrapping the libraries in the treble API. So it actually uses IPC where both sides are in user space. That's what they mean when they describe it as 'in process' in the documentation.

Going forward, I expect the vendors to modify their kernel drivers to export the treble API over binder IPC directly while in kernel space.

So I wouldn't be surprised if it ends up that interprocess RPC only ebds up getting used in systems before Oreo.