https://www.seeedstudio.com/Sipeed-MAIX-I-module-WiFi-versio...
How are processes isolated from one another if there is no MMU? Is there some other form of memory protection? That doesn't seem likely?
Not having process isolation really limits the applicability of the chip.
This looks like to be a bare-metal (or RTOS) embedded system. It usually is for running a single application so isolation between processes doesn't make much sense. FreeRTOS doesn't even have concept of process neither, but tasks, which is similar to threads with preemption and deadline.
Furthermore MMU is often avoided in embedded systems because it violates hard real-time requirements: Translation table walking can take place while an interrupt occurs so it causes unpredictable worst-case execution time.
In case of ARM Cortex-M and R series they have Memory Protection Unit (MPU) instead of MMU to protect critical memory region.
(FreeBSD has RiscV support for versions with an MMU.)