This article, in the Kurdish ARA News, raises a few considerations:
GEN Votel visited Kobani the very day after Turkish backed rebels threatened to attack Manbij.
One purpose of attacking Manbij could have been to draw Kurdish forces away from the Raqqa offensive, and delay the fall of Raqqa.
This objective may have been a key factor in the ISIS garrison of al Bab agreeing to turn the town over to the Turks - they expected heavy support to recapture their former stronghold of Manbij (under a different false flag).
Interesting how the Turks don’t seem to have a bunch of ISIS prisoners from al Bab, or any preparations for holding trials - they seem to have just joined forces, or been allowed to leave.
My understanding was they left. Don’t recall where they went, but it was to the east if I remember correctly.
I think I could look at a map and figure it out, if you like.