>I understand from another comment that this is intended for embedded systems.

Nope, he said embedding. He means things like Lua where you have a scripting language embedded in a program that is running on a faster lnaguage. Common in video game dev.

>Does that mean running it on a microcontroller without OS?

Nope, not necessarily. It being portable means it's plausible but it's an unlikely usage scenario. More likely, it means being able to use it across a variety of POSIX derivatives.

>Cause the moment you can flash Linux, you can use Ruby MRI, no?

Also not true. What a micro-distribution of Linux can run on is not necessarily something MRI Ruby can also run on.