>After the Great War, the Thalmor and Ulfric established direct and cooperative contact, following which he prove his worth as asset to the Thalmor.[11] The so-called Markarth Incident[11] resulted in the Thalmor being allowed into Skyrim to enforce the White-Gold Concordat,[9][16][17] however, since it also imprisoned Ulfric he turned uncooperative.[11]
This just comes from the left field. I thoroughly inspected the sorces and there is not a single hint reinforcing quoted statements.
Ulfric indeed was captured, tortured, got broken and spat some long outdated to that point info. However, there are no cooperation. Even more, the only way Ulfric helps the Thalmor is by pulling Empire in the war of endurance. As Thalmors themselves point out in their reports, no matter which side wins, quick win by either side is horrible outcome for them.