The kernelsprite only turns into a full sprite after entering the Medium, so yes, this image is impossible.

I speculated in another thread that the reason we don't see this room ever again is because it's actually one of the ones Terezi adds on to Karkat's hive over the course of the game, which fits with it being clearly post-entry and being empty + light grey. But I myself never thought of that until that post, so it's not really a surprise that the animators thought of it as an alt version of Karkat's respiteblock, and thus drew him "hanging out" in it on Alternia. But yeah lol