I'm paraphrasing.
It would nullify the results of the testing if anything changes other than the tyres Pirelli supply.
Ultimately, whatever tests Pirelli tell them to do, Merc will be getting telemetry based on the performance of that tyre, they will surely know the tyre compounds and ultimately then can use that data when they are used in the next race...
Lets put it this way....
Why would a top team invest time and resources to run a test if there was nothing in it for them? Even Ferrari will have benefitted from their test albeit in a car that is a few years old. The data can still be somehow used to their advantage...