Is it all that unreasonable? I don't see that Google's Zircon kernel has API guarantees either, but as the driver model doesn't require mainlining, it suggests the code can remain under wraps and ostensibly kernel specific.
If Zircon is a proper microkernel system, then it ought to have such guarantees. But it's too early to tell anything about Zircon and at what end of the microkernel spectrum it will land.