I see some nonsense is being talked. Running a 2012 diffuser is not what is being stated here. What is is that running such an exhaust with a diffuser designed for a completely different configuration will make the car inherently unstable and not really tell you anything, or at least it should if it was working properly beforehand. There are two possibilities there.
If people had actually read what Gary Anderson had stated he said that the new exhaust configuration is not unknown to the teams, being what they ran prior to Red Bull starting the whole exhaust thing off, so at least getting something on the car to work well enough with what they were running would not have been too difficult. There is nothing about running a 2012 diffuser here. It's a logical thing to be wondering.