It was probably meant as they give the same answer as things are at the moment, but they wouldn't if the number of blocks per unit of time would be increased significantly.
Right, with a 10 minute block time, there are so few orphans that there is total agreement.
If the block time was 1 second, then there would be a huge number of orphans.
Some system for quickly finding the leaf block is important. Each time a header is added, you have to update an entire tree.
An analysis of the optimal strategy for miners under GHOST would be important.
For bitcoin, broadcasting new blocks as fast as possible is optimal. Is that still true for GHOST.
With GHOST, there would be lots of ties and near ties, so colluding to break ties in favour of a group may give those in the group an advantage.