First of all don't forget its 2007 trip 
And, dont get confused about that. There are no two thoughts. Deosai is a pure 4x4 hardcore jeep track. There are many muddy patches and water stream crossings where only jeeps can cross. Moreover, the way to Skardu just after exiting Deosai National Park is a dangerous jeep track downhill.
The road from KKH to Astore to Chillam Chauki is manageable with cars as well but not Deosai. And also once you go down to valley that leads to Sadpara Lake and Skardu, that can also be done with a car but climb up to Deosai from either side and then the drive within Deosai is only jeepable.