BTW, Guide to Hell is an odd case in 2e.
It did not bear the Logo of any of D&Ds (meta-)Settings and did not fit with what the Rest of D&D worlds had firmly etablished between them.
While some of it's ideas eventually found their way into the Rest of D&D, mostly in Form of unproven rumors, i'd hestitate to use that one as a real source for anything
Shannon Appelcline's
history for that product provides some interesting commentary, here. The book didn't bear any campaign world logos because at that point Wizards of the Coast was very concerned with the perceived factionalization of D&D's fan base. To that end, not publishing the book under a campaign world logo was becoming routine, as books like
Carnival (for Ravenloft),
Warriors of Heaven (Planescape), and
The Vortex of Madness (also Planescape) also had no campaign world logos.
Also, I'm not sure why you're suggesting that the
Guide to Hell didn't fit with "what the rest of [the] D&D worlds had firmly established." It added new lore regarding Asmodeus (and Jazirian), but it otherwise remained very true with regards to what Planescape had established at the time in its capacity as a sort of meta-setting.
For example, Planescape lore had previously noted how, although there were tieflings of baatezu heritage, there hadn't been any known half-devils; we finally had them make their debut here.
Oh Hallowed Ground had mentioned how people who died not believing in any sort of afterlife whatsoever were simply gone, not appearing as a petitioner anywhere. This book revealed that they reappeared at the very bottom of Hell, where Asmodeus consumed their souls. It also referenced the Flight of the Fiends from Greyhawk, and how that had only worked on demons (which fit with the lore, since the Flight had been concerned primarily with the demon forces of Iuz).
Overall, the book comes across very well with regards to how it fits into the picture of AD&D's view of the planes.
