I asked in the Technical Support subforum and QuintusSertorius answered that in the campaign script, there's an ultimate fallback section that triggers the Polybian reforms no matter what by turn 254. It uses the Rebel turn FYI. If you don't want to wait that long, trigger a reemergent faction revolt in one of Rome's starting regions and wait at least one turn for the turn 200 AI trigger to activate. Then of course quash the reemerging Romans.