Mercedes have excellent tyre warmup, so they likely build the correct temp up on the formation lap.
The FIA are obviously quite happy with the explanation given that the temperature of the tyre was lower than all other cars when the reading was taken therefore the pressures would be low. Soon as they get temp into the tyre its totally legal and its the process of when, where and how they check these new minimum pressures thats the issue. They admitted as much when they say:
That's basically an admission that "hey, we didn't actually state when we would check the minimums but we will in the future", be that as the cars leaves the garage, on the grid or whatever.