The problem with that idea is that there was no way for Ruby to have knowingly timed his arrival to coincide with Oswald’s departure. IIRC, Oswald was scheduled to be transferred earlier but the move was delayed, I believe by the huge media crowd in the building.
Inside job? He may have been told when to be there. I was in fourth grade at that time and was PISSED that my Saturday cartoons were not on. I was watching all of this live when Ruby came through the crowd and shoved a gun into Oswald’s gut and pulled the trigger!