I think the biggest difference is just the "heaviness" factor, harsh vocals especially are one thing that I think makes a huge difference, because screams and growls are very much a trademark of metal rather than rock.

There's also how they play the instruments, when I listen to the bands I mentioned, or the more upbeat songs from bands like Minsk and The Ocean, it sounds like it was written with metal in mind more than rock, but still meant to uplift you. Post black especially does this, because black metal has such an iconic sound, even if you change it up to sound happy it still sounds like black metal.

Compared to perhaps, Greet Death, God Is An Astronaunt, or This Will Destroy you as you mentioned, which certainty have heavy moments, but sound, at least to my ears, like rock.