No, he's right.
Usually, "ripe" tadpoles are sent through ceremorphosis at the very first opportunity - everything else would be blasphemy. There is no delay in that - it isn't happening that often, anyway. What you say is exactly the opposite. Those tadpoles that aren't "eaten" by the elder brain are the one that are turned into mind flayers.
But there can be extreme circumstances, usually the destruction of a illithid colony, inlcuding the elder brain. When that happens, the tadpoles aren't fed any more, so they become cannibals, and only one of all these tadpoles survives it. It becomes really large because of the psychic energy it absorbs from all the others. Finally, it has to leave the pool and go into the greater world, eating rats and vermin, growing even larger.
Finally, it eats a sentient creature, and from that first intelligent brain, the latent psionic powers awake. Only then it is really a neothelid. Before that, it's just an ugly, oversize tadpole.