All modern kernels provide the underpinnings that a smart watch requires and stripped down would have similar memory and power footprints. There is nothing special about QNX which would make it magically work better than the same watch running a Mach, Linux, NT or some other kernel.
What matters *far* more is what the operating system above the kernel is doing with itself and the active display on the device. The display alone might eat up 80-90% of the power.