This 6:35 laptime is nothing more than a rumor. Maybe a convenient leak from Ferrari who is trying to gauge what their rival at McLaren is doing. They both seem to be waiting on each other and they both know the Nurburgring is a measuring stick. Essentially, they each want to beat the other.
Let's say this mind blowing 6:35 is accurate. Well, McLaren would then know what target they have to hit. Let's say McLaren then comes back with a 6:32. Ferrari would then know they need to go to 6:31 or better. And so on, and so forth.
At some point somebody is going to have to release an official time. That said, Ferrari seems to be keeping the LaFerrari XX in reserve in order to maintain perceived superiority. Yes, it is based on the production LaFerrari but the XX is not a street legal car. It is a turn key race car with factory support for the few Ferrari invites to have the opportunity to purchase it.
Even if the P1 beast the LaFerrari around the 'Ring Ferrari can then use the XX to claim they are faster. It's not a stupid move.