Is it? I'm not so sure. Certainly the presence of LittleKernel, a lightweight real-time kernel suggests it's an RTOS but that kernel is wrapped by something called 'Magenta' which says "Magenta targets modern phones and modern personal computers with fast processors, non-trivial amounts of ram with arbitrary peripherals doing open ended computation." [1]
[1] https://github.com/fuchsia-mirror/magenta/blob/master/docs/m...