Astarion is just a vampire spawn, and hasn't been allowed to drink human blood by his master. Plus, the tadpole.
People keep saying that like it means something. Curse of Strahd has many vampire spawn that didn't drink any blood at all, probably for decades or centuries, and they're as powerful as you would expect the vampire spawn to be (CR 5, just slightly lower than a mind flayer).
Tadpole is a dumb plot device. It depowers Gale, whose power is born from his mind, Wyll, whose power is born from the infernal pact, Karlach, whose power is born of her natural strength and skill, and Astarion, whose power is born of his innate physiology - somehow, it's capable of targeting every possible power source you can imagine. But Lae'zel is fine. She was never strong before, shouldn't the tadpole weaken her even further, if it made Karlach from a demon-slaying machine into a weakling? Shouldn't Shadowheart be completely powerless, if Wyll was reduced from a powerful, legendary hero to Wyll? Why are these two specific people fine? And why do the companions at no point frame the tadpole problem as "Ooooh, if only there was no tadpole in my brain, I could solve this whole cult problem by myself"? Why is Astarion (and everyone else, should you convince them to use it) treating the tadpole as a source of power, as opposed to a source of weakness? Is it because the writers know it's a shit excuse and don't want to call any attention to it?