But, I think it cannot be Microkernel. Because, it dose not have the "User Mode", so it could not separate kernel and driver. And, ChibiOS's drivers are include same binary.
While it does not usually implement a kernel/user space separation it has been used also that way. You can do separation on top of RT, it is just not really required for an OS running on MCUs.
An example is ChibiOS/RT running in trust zone on a Cortex-A with a Linux kernel running in non-trusted space, the two OSes can communicate using an API.