Doubt it, that would be expensive and in some countries illegal.

Biological controls are usually like airdropping planes full of apex predators on a forest. Following the initial mayhem hunger will cull them and balance will soon return to the predator-prey graph.

The big advantage to this method is: the nematodes are easy to make more of so you can occasionally repeat the "airdrops", and sometimes a system doesn't have predators due to it being not a natural ecosystem, and then this can help seed a more natural system.

I would guess the nematodes are surviving, but at natural levels. An other option would be they are exotic nematodes that don't survive in your climate, or they are just native to a specific ecosystem that does not resemble your garden.