When the oblivion crisis occurred, all provinces had gates opening up. Due to the altmers understanding of oblivion and realms of the daedra, altmer warriors knew how to close the gate from the beginning.
What is your basis for claiming that the Altmer warriors knew how to close Oblivion gates from the beginning? Notwithstanding that, it's not exactly rocket science as an Imperial Legionnaire named Menion Goneld is the one that tells the Champion of Cyrodiil how to close an Oblivion Gate.
It's one thing to know how to close an Oblivion Gate and another entirely to actually be able to do so. The Altmer of Summerset Isle certainly didn't display any notable ability to do so as the Daedra spilled forth from multiple Oblivion Gates and quickly overran the land driving the survivors to seek refuge within the Crystal Tower which was subsequently overrun and destroyed by the very same hordes of Daedra (as described in Volume I of
Rising Threat).
Aside from that if you played The Elder Scrolls IV: Oblivion then you have to know that all the Oblivion Gates were sealed when Martin Septim as the Avatar of Akatosh defeated Mehrunes Dagon. The Altmer in general, and the Thalmor in particular, had absolutely nothing to do with the sealing of the Oblivion Gates in Summerset Isle. What the volumes of
Rising Threat chronicle is the Thalmor's rise to power through the use of lies, deceit and manipulation and it's implied by the author that the Thalmor claim to having ended the threat of the Oblivion Gates was a lie to elevate their status in Summerset Isle in their bid to take over the province:
Rising Threat Volume II said:
Before my numbed mind could comprehend the tumult that consumed my beloved Summerset Isle, before I could formulate the question "how?" they were there,
dripping honeyed poison in our ears: the Thalmor. They were the ones that saved us,
they claimed, working deep and subtle magicks. It was their efforts, their sacrifices that delivered the Altmer from extinction.
Aside from the game content from ES IV clearly refuting their claim, players who have played the game know that the concept that you shut down Oblivion Gates with "subtle magic" is absolute horpl***. The Thalmor clearly took advantage of the situation and lied to the Altmer of Summerset Isle to seize power.