Is the dialog you have with Serrit a lie?

Spoiler ahead.

When you find Serrit in his own blood, critically injured by Sile, and have a discussion with him, he says something along the lines of "Can't you see? We could have ended the Wild Hunt curse once and for all, stop the Omen of War".

Now, I find this affirmation quite interesting but unfortunately the game doesn't explain more. What does it mean? And most important, how did they intend to do that? Seems like a lie, since Letho admits that all their actions were done at the emperor's command to make the Northern Kingdoms weaker. But is it a lie? Are there any references to this in the game or the books that may shed some light on the matter.

^{Spoilers! Duh...}

Serrit is probably referring to the fact that the three Viper witchers and Geralt fought the Wild Hunt together.

If I interpret this video (starting at 13:32) correctly, Serrit is talking about the time when Geralt was looking for Yennefer, who had been abducted by the Wild Hunt (in order to draw out Ciri).

It's explained in this video:

Geralt, Letho, Serrit, and the 3rd witcher from the Viper school found the Wild Hunt, and slaughtered a lot of them. they couldn't kill them all (which is one thing Serrit is referring to), so Geralt offered his soul in place of Yennefer's.

"The curse" of the Wild Hunt isn't that much of a curse per se, it's just that those buggers go around kidnapping people for their own purposes, and they're also tied to Ragnarök, which not even Viper School witchers want.

Sile knew, about the Wild Hunt and Yennefer, but she didn't want Geralt to learn. Why? Because Geralt and Yennefer's love was an important source of trouble for the sorcery lot, and right now Geralt was happy in believing that Triss is the love of his life and always been. Was Geralt to start hunting for Yennefer (hallo, Witcher 3), this would create a lot of complications for their "plans".

How did Sile know? Probably because of the Lodge of Sorceresses - they had a Nilfgaardian sorceress, Assire var Anahyd, who could inform them that Yennefer was in Nilfgaard. While initially reluctant to tell this to Geralt, Sile changed her mind in the end of the game (assuming you saved her, of course, and even then we already knew that from Letho).