The thymol I believe destroys the nosema spores in the hive through it's antiseptic properties.
Fumidol B was a prophylactic, in that having picked up nosema, the Fumidol acted to prevent the spores from breaching the gut wall and multiplying, sort of a spore bypass mechanism if you like.
Once again a pricey option that mitigated the effects of nosema, but didn't actually cure it. As is often mentioned, as the weather warms up bees tend to shake off nosema, or at least the effects of it. Thymol appears to be the answer and big business knows how to package it and get their products approved so that you can empty your wallets in their direction for not using the generic product.