Thanks! The article never mentions the direction of the MW's spin, so the bias section reads pretty roughly. If the reader knows that going in then it makes perfect sense:

• MW spins counter-clockwise

• Most observed galaxies spin clockwise

• Doppler effect would make galaxies spinning the opposite direction of Milky Way appear slightly brighter to us,

which suggests observation bias as a possible explanation. I only got 2/3.

A strategic "also" in "This could explain why such galaxies seem more common in telescope observations." would have helped as well.