My guess is his old ‘host’ knows Oliver didn’t really murder Sara, and gives him the dip to bring him back.
Given that while Arrow as a series is in the crunchier/realistic end of the DC universe, this might seem a cheat. Except that Flash has been in play as well. So there’s no reason to believe the Pits couldn’t exist.
Seeing as the rumor mill says that the 1st 3 post-break episodes will be Canary-centric, I think this will give Star City a chance to break again while its protector is away. The 1st half of the season was rather introspective. And at times that might have seemed problematic. But I think it paid off in the end, as explanation for why Oliver would take Thea’s place.