It's both; the one I mentioned is for system drivers, the one you're talking about is for running applications (which you can also do on a regular non-Halium GNU/Linux using e.g. Waydroid).
I'm sure that if you tell Jolla about a relatively modern mobile SOC with mainline linux support, they'll look into it instead of relying on libhybris.
They can rely on libhybris if they want, why should I care - I just object to calling that "a full-stack alternative", especially when alternatives do exist.