So Altansar is pulled into the warp and... then it comes back out a few thousand years later. We see they still have crappy guardians, like anyone else, because that horrible magenta colorscheme shows up in the 4e codex.
Did they 'covertly' get sucked into the Eye and no chaos marines noticed? None of the 4 gods noticed? No demons casually noticed?
We got fluff of a single keeper of secrets annihilating a craftworld. We got fluff single marine chapters destroying craftworlds. We got fluff of tyranid swarms accidentally bumping into craftworlds and starting a fight...
So how the heck did Altansar not get eaten by an army of demons immediately?