I'd go by the general stance that "through" means straight through a route without looping back.

Whether you do that in a leisurely fashion, stick to the set route religiously or do a bunch of side trips is immaterial. It's not a loop, not a lollipop, not returning on foot to the car, it's a thru-hike.

I'm building my own thru-hikes. Will be dropped off and picked up on either end due to the remoteness of the trailheads involved.