I think it reasonable doubt for that. The Ulfric dossier specify that if the Empire win then Civil War their position in Skyrim will be weakened. It doesn't prove anything, but it implies the Thalmor isn't as influencial as one might think.
It also says that a Stormcloak victory is undesirable. They want chaos, not a victory from either side.
As long as the civil war proceeds in its current indecisive fashion, we should remain hands-off. The incident at Helgen is an example where an exception had to be made - obviously Ulfric's death would have dramatically increased the chance of an Imperial victory and thus harmed our overall position in Skyrim. (NOTE: The coincidental intervention of the dragon at Helgen is still under scrutiny. The obvious conclusion is that whoever is behind the dragons also has an interest in the continuation of the war, but we should not assume therefore that their goals align with our own.) A Stormcloak victory is also to be avoided, however, so even indirect aid to the Stormcloaks must be carefully managed.
The emperor is turning a blind eye and he is the most important person in the empire and he matters most as he is the supposed ruler of the empire.
Ulfric finds it more prudent to spend resources on his personal war than joining the empire and facing a greater threat.
Those people are of little consequence under their leaders, that is why we need somebody competent on the throne who can drive out the Thalmor and crush them. Then the empire could regain Valenwood and the Summerset.
Likewise the empire considers it a good idea to bleed their own people, even use the Thalmor against them, than to recognize Ulfric's victory in the duel with Torygg and let Skyrim secede as they let Hammerfell and the other provinces go.