With the post mortem completed, the verdict is that Ferrari struggled to keep the battery charged at Spielberg, as there were few braking zones which meant super-clipping was the main way to recharge. As such, Ferrari had to spend more time super-clipping than the Mercedes, Red Bull or McLaren, resulting in them running 20kph+ slower towards the end of the straight and making it much easier to pass.
Unfortunately for Ferrari, Silverstone is similar to the Red Bull Ring in terms of energy management, so expect them to struggle there, as well. On the flip side, Vasseur admitted that they should have accepted they were not going to be able to keep up with the Mercedes and focused on keeping McLaren at bay. So I have a feeling they will go in knowing that at best they are fighting for third place.