The scooters have very poor durability. They're lasting on the order of 100-200 miles in rental fleets. The study actually underestimates this effect and assumes that they last about 5x longer than they empirically do. Bird and the scooter manufacturers are trying to address this by making beefier scooters that don't fold with bigger batteries, but the abuse they take in a dockless rental fleet and from having such small wheels make durability a big challenge.

A moped typically lasts 10,000-100,000 miles, so even though it's much heavier, the energy and cost are amortized over a far longer period.