AIUI there's only one such preemption point, in some resume-able operation to do with cleanup. Every other operation is short enough.

Thus interrupt latency is guaranteed to be very low, even if the interrupt happens while the microkernel is running.

It is my intuition that the "it's not a RTOS" has to come from something else, like some non-implemented API that's expected in an RTOS or the like. The required services might actually be implementable as non-privileged tasks.

Maybe it's just slight conservatism. If all existing RTOS' have pre-emptible kernels and yours doesn't, perhaps you'd rather let the market decide you've built an RTOS rather than stating you have and risking an argument over terminology.