While I'm at first inclined to say, no, it's not on-topic on G&L, the presence of hookworms in the soil could be a problem just as much as say, yellow jackets (and not just for pets). However, hookworms probably aren't something people usually think about in a gardening and landscaping context, since they don't typically interfere with gardening and landscaping (unless you don't like to wear shoes, or unless the dog is part of your landscape, in the same sense as a pond of fish). The hookworms are part of the landscape, though (much as heavy metals in the soil).
I'd say it would be okay to ask here, but it would be unusual, and potentially get close votes. I would think it would be more on-topic on the Pets network, especially since the focus of the question is the dog.
Your answer to the question demonstrates why it might be a better fit for the Pets network. That is, the problem might not be the soil at all, but the parasite lingering and reinfecting through things the medication didn't kill.
The bigger question seems to be about eliminating parasites generally, rather than this specific question. If it's about eliminating them from a non-landscape pet, I'd say no. If it's about eliminating them from the soil in order to protect a pet, I'd say it would be unusual, but slightly on-topic, and a better fit for the Pets network. If it's just to eliminate them from the soil without regard to a pet, I'd say it would be unusual, but on-topic.
If it's about removing parasites from plants, I'd say it's on-topic. Some plants even are parasites.
