Seems to be a spin-off of the PLAN C episode. I always had the suspicion that those people ended up in a boundary between the normal world and a hell-like dimension as they simultaneously achieved a state of half-conscious, half-unconscious - but also half-dead (in case you forgot, the group in that episode was apparently doing a suicide pact with very similar methods to the family in this chapter). In this chapter, Miichan also makes a reference to "sleeping while being awake" which to me sounds like an intentional callback to that episode's circumstances. And the last manga drawn by "Yuu-chan" even shows their car heading towards the dark swirling portal that is not explicitly mentioned, but alluded to in PLAN C.
The primary difference was that the group of people in PLAN C completely vanished, seemingly having been spirited away by the residents of the other world as their screams (and the demonic beings' grunts) can be heard in the final seconds of the recording. But then we see that Sachi was drawn like she has horns - perhaps this was meant to signify that she was possessed by the other world along the way. In PLAN C, there was also one guy who suspiciously sounded like he was the one sacrificing everyone including himself.