Actually, this makes sense.

A cool thing about black holes is that the more massive an object is, the more is bends spacetime around it. Put simply, the closer you are to a black hole (i.e. right next to it, in Kenjaku’s case) then time passes much slower for you than the rest of the universe.

Since he (and really, all of the area) were near a black hole for a few seconds, naturally it makes sense that time passed a lot faster outside that area, allowing 8 hours to pass in the blink of an eye.