That was exactly my thought. It's like DDT on Mosquitoes. The ones that survived were resistant, so when they bred, all their offspring were resistant.
Could end up being the same issue here, so this could easily become a FAR FAR worse issue in the next generation or two of these worms.
My brother worked in a mosquito lab for a while and said that mosquitos lose these resistances pretty quickly (as a population) if they aren't regularly being dosed. In the absence of pesticide, the more efficient organism is generally the one without whatever metabolic fluke made it immune to the poison.
The lab helped inform the city's strategy for fighting mosquitos, which was to rotate pesticides. I believe they also bred and released neutered mosquitos.
Could end up being the same issue here, so this could easily become a FAR FAR worse issue in the next generation or two of these worms.