This is frankly the first i’ve heard of RIOT, and I’ve been doing IoT products for more than a decade. It sounds intriguing, although i’m always suspicious when an embedded platform claims to be “just regular programming” — that usually means your treadmill or wristwatch will crash as often as your Windows 95 machine used to. Embedded products use a different methodology, which is why your car, TV set or thermostat so rarely gets a BSOD.
Probably the most popular open RTOS in the IoT, or embedded devices in general, is really FreeRTOS. Some chip vendors make royalty free RTOSes like TI-RTOS.