For me, it looks like F2Pool is trying to win time until another significant move in the blocksize debate happens (UASF, or a possibility of some kind of "Flippening" that puts in danger their business model).

It looks like they are totally OK with the actual situation (relatively full blocks, high fees) and want to extend it until there is absolutely no way to not support one of the antagonic factions. If that is true, they can safely signal support for extension blocks because this proposal is not backed by other of the larger mining pools. Maybe they are even truely OK with extension blocks, but for now perhaps that doesn't matter, it may be simply part of their strategy to conserve status quo.