I see the logic in that, but isn't the safe bet to sky train the whole stage? Dumoulin will have to attack regardless.
I'd imagine they will want to put time into Dumoulin. Thomas is still largely unknown at this stage of a GT, it's unknown how Froome will respond after the Giro, and the world TT champ isn't 'that' far behind. They'll want a buffer imo because at the moment if something happens to Thomas, Dumoulin wins because he can take time up on Froome in the TT.