Michael didn't edit any of his short films, post-production teams were hired to handle that side of the film and amongst them, an actual offline editor who'd cut the film together. What he probably means is that he is very influential in the editing process, having final say.
Similar to what KOPV described, Michael likely assumed the position of the director (along with the actual director, who would be there too). I imagine most days went like this ... he'd come in with the director, they'd discuss what had been done so far since their last visitor, editor might discuss any issues with the footage and possible solutions, MJ and the director give their feedback, some pointers and specific things they want to happen/express, then they'd leave and come back at a later time.
This is assuming he wasn't busy at the time, like on a world tour in which case he might not have been as proactive and left more things up to the director/editor.
