Ivermectin can be used to treat hookworm, but it's not considered the "best" initial option. There is a reason that Ivermectin is not approved for use in Canada - it is an extremely potent anti-parasitic that is not well tolerated by some people with a genetic predisposition for complications. Dosages are calculated in small fractions of a mg per kg of body weight. An accidental overdose can kill a person and there is no easy way to flush the drug from your system in such an instance.
Ivermectin is generally used with large farm animals and here in the DR is often prescribed by veterinarians for de-worming dogs. Obviously I can't second guess the doctor in Canada but the two primary medications usually prescribed to kill off hookworm are readily available from pharmacies, effective, come with a low risk of side effects and are relatively cheap in price. Curious that neither of these two medications were tried first before going the unapproved drug route.
Moral of the story, when you travel, you expose yourself to the risk of contracting diseases and parasites that most doctors at home have never seen and may find difficult to diagnose and treat.
Had the more serious symptoms presented themselves here in the DR, perhaps the identification and treatment would have been a tad more straightforward for this couple. If you find hookworm gross, don't try swimming in a dirty river here...