Using the OS is allowed. As far as I know, one of the reasons why there are more Windows demos than for example Linux, is that Windows provides more graphics/sound APIs out-of-the-box while on Linux the OS itself does less you'd have to utilize libraries (which is kind of cheating when doing a 4k demo).
I think it could be summarized as "write a demo for the target platform as efficiently as possible". Doesn't necessarily need to be bare metal, there are JS demos that run in the browser as well. In that case the browser (with all its APIs) is the target platform.
I think it could be summarized as "write a demo for the target platform as efficiently as possible". Doesn't necessarily need to be bare metal, there are JS demos that run in the browser as well. In that case the browser (with all its APIs) is the target platform.