That is stated, but I’m not sure I’d consider the Netherbrain a reliable narrator. I feel like it was an attempt to intimidate the party. The sheer amount of secret knowledge and unlikely events that come together to begin freeing the brain at the end of A1 seems beyond the ability of an elder brain, which is all it would’ve been when it formed the plan.
The whole idea that it would use use an artifact that had only been seen once, very briefly, with no measurement or understanding of its true capabilities, to evolve itself into a being powerful enough to overcome the control it was submitting to as part of its plan… I know elder brains are scary smart, but this seems a ludicrous proposition.
Consider: what part of its plan could it not have achieved simply by tadpoling Gortash and anyone else to begin with?
Now it could be that what the brain says is canonically true and this is just weaker writing, but until I see confirmation I think the brain was trying to demoralize the party into thinking they’d always been dancing to the brain’s tune the whole time and everything they had done had been in service to its plot the whole time, when the truth is it has been (very capably) trying to regain the upper hand since Ketheric’s death.