My worst infestation came from purchased bags of soil that were stored outside and had been rained on.
I usually mix my own, but that one time.. all it took to know, Do not buy soil mix from the parking lot of the Farm store.
It seems the "Tanlin" is just Xanthan gum. A common food additive used in medicines and vitamins as well. Even the manufacturers website didn't provide an exact mode of action that I could find. I was really perplexed as to how this stuff works.
WebMD had the best info about it. From what I read, the only thing I could see as mode of action was similar to BTi. Once larvae ingest it must stick in their intestines and swell. Maybe making them constipated to the point of not eating?.

Miserable Bastids, that's what they get.