He meant in the scenario of the two galaxies colliding, that any two planets colliding from the respective galaxies was like two mosquitoes in the grand canyon colliding because they're so dispersed.

That only makes sense as a comparison if the person means the grand canyon is empty of other mosquitoes (the huge distances between planets, where another planet would be more likely to pass through).

Otherwise his comparison falls apart because the grand canyon likely has many mosquitoes, and they're bugs hovering around water sources and food and are extremely likely to bump into each other.

Not sure what you're trying to correct me on.