The author assumes the dogs weren’t transported to both Mexico and Peru from some other location(s). But he doesn’t assume they’ve never been found anywhere else. What if they were transported from Mexico to Peru, and then to Argentina? That wouldn’t damage the argument.

Too strong. The author doesn’t need to assume that most trade from all places to Mexico was by boat. He just needs to assume that there were at least some trade expeditions running between Mexico and Peru.

Too strong. The author doesn’t need to assume that all boat travel was for trading expeditions—just that at least some boat travel was (enough to make it likely that the dogs were on some of those expeditions).

Whether the dogs were traded during trade expeditions is irrelevant. The author merely assumes that there was some way for the dogs to be present on those expeditions. Perhaps they were brought as gifts rather than exchanged, or perhaps they were there by accident.

The author implies a comparison between overland and boat travel but never says how difficult boat travel was. He must assume boat travel was easier. Otherwise, if it was just as hard or harder than overland travel, the conclusion becomes unsupported or even anti-supported.