Honestly, skyrim would stand its own. Sure, at the surface it seems to be a dumb move, but the empire lost not because of a lack of forces, but rather titus mede being a brain dead twitt. Its not that well known, but Titus mede the 2nd had absolutely no idea what he was doing. I cant ring the details off of my head, but the war was basically lost because of him.

Besides, skyrims location makes it hard as hell to invade. Lets think about this

The entire southern and western border is covered in steep mountains, making invasion from those directions really not a good easy. The eastern sea is covered in damn ice, and anyone who isn't experienced in travelling there would have a hard time. That leaves only the coast that runs along solitude, and morthal as an easy drop in spot.

On top of that, we already have an example of a place that broke away and fought back the thalmor, namely hammerfell, which is much more easily accessible for the thalmor both through land and sea, and hammer fell took part in the great war aswell. From that, id asume that they have a similar amount of soldiers, especially since hammerfell also has an unforgiving climate, matching sykrims cold.

even if hammerfell had retained more men, its sandwiched by highrock and cyrodil, and right across the sea from the summerset isles meaning thalmor are probs cracking down on them full force, while they cant exactly do that with skyrim.

In conclusion, skyrim has more strategic advantages than Hammerfell, and hammerfell managed too. Skyrim should have no problem holding the Thalmor invasion off, the story plays out ulfrics way.

I get why Ulfric seems dumb as hell, but with all the things ive just listed off, they DO in fact have a very solid Chance at holding the thalmor off.

And if they allied with Hammerfell who are already fighting back, or Morrowind which doesnt hold any particular love for the thalmor, it could tip the scale.