Sentinel treats heartworm, roundworm, whipworm, hookworm. Strongyloides, or threadworm, is considered a roundworm. The active ingredient in Sentinel is Ivermectin, which is the best treatment for this worm. You have good advice from your vet.
Worms, Threadworms (Strongyloides) Symptoms and Treatments in Dogs | Pet Health at Wiki-pet.com