From an in-character perspective, we have a character that knows far more than most people in the setting about the tadpoles presenting a definite way to get rid of them. It's not even like she's vague about the method, she describes a process of removal and and I didn't encounter any reason to suspect she was lying at all in my playthrough. So from an in-character perspective, her solution is as close to a guarantee as we get.
Just started another playthrough (hopefully not going to finish it due to patch4 arriving :P) - THERE ABSOLUTELY IS a reason not to trust Lae'zel: In the conversation after her interrogation of Zorru, you can peer into her thoughts ([WISDOM]) and it becomes clear she is hiding something regarding the purification, so, yeah...
My guess it's going to be something like: "They are going to perform the purification on me (maybe), but you slaves are going to be fed to the dragons."
Buy it might be something else regarding the purification.
It's sort of a reason not to trust Lae'zel, but she has proven to know the most about Mindflayers and has a lead to deal with the tadpoles. (Gale knows around as much, but does not provide a lead.) So she isn't lying, and I don't think she's being deceiving - she's a pretty straight forward character. But I think what she is hiding is that she is unsure of the process herself, whether they are even worthy of it, or if purification itself is just death itself. She has the same look during the Gith Patrol scene when she is unsure of what's going down.
I also don't think she's pulling all this out of thin air because she mentioned she studied extensively in the libraries of her creche.
It's like, it may be a good lead, and I don't have any reason to believe she's lying, but it's best to prepare for it as a tough road to go down. Which is also an in game reason to yes, understand Lae'zel's urgency, but to also know that you shouldn't run into it without being prepared. At present, it is too easy to make the mistake of making a bee line to the Gith Patrol scene, only to get wrecked as a new player. It would be nice if they could add some more overt element that you should probably gain some experience under your belt before going that route.