The irony of this "advice" is that he recommends the slow kill method with 5-days of doxycycline every month. Right now, that costs $40-50--just for the doxy (!!!)--every month for 18 months. So, $40 x 18 = $720, if the Doxy prices don't stabilize and come down soon.
At that cost, you might as well just do immiticide (fast kill) treatment ($500-$1500, depending on where you live and what vet you use), and be done with the exercise restriction and get the dog healthy in 2 months. ETA: I also think his advice about slow-klll is out-dated. Not only has the AHS come out clearly and strongly in favor of using immiticide (instead of "slow kill"), but even when slow kill is used, some vets where I am are more likely to use Advantage Multi than ivermectin as it seems to be clearing the worms faster than ivermectin, from what I'm hearing in my rescue dog circles around town.
This sort of advice about using preventatives only seasonally tends to be given by people who haven't ever had to treat a HW positive dog. They roll the dice and on the odds their dog will be safe. Some of them will be, some won't be. It is, after all, playing the odds. If you end up unlucky, you've got a $1,000 vet bill, and a dog who will be on exercise-restriction for at months.
Also, I've known several people who did the ivermectin slow-kill method of killing worms, and their dogs still tested positive at 18 months. 24 months is not uncommon, from what I've heard. That's 2 years of additional damage to the heart and lungs. No thanks.
Having taken 2 dogs through fast-kill HW treatment, it's not something I'd ever knowingly risk putting a dog through, if it could be avoided. It's just not worth it, when it costs just $5/month for generic heartworm preventative pills -- it's so cheap to keep them on it year round, if you buy a generic online or from Walmart's pharmacy. I just don't get the resistance to it.
BTW, I believe AHS is now recommending year-round preventative, according to what I read a few weeks ago on their site.