It’s a very positive move on many levels. Not least, battery power might allow a bus with a low floor throughout rather than requiring a big step-up at the rear, over the engine. Space for batteries can be fitted in other places that work better with the seating.
It will also be positive to reduce fumes and noise in the urban area.
However “as with a Tesla, is that these are 100% emission-free” is highly misleading. Even if the electricity were generated entirely from wind or sun, there is an embodied environmental cost in the bus, the solar panels, the wind turbines and all the electrical infrastructure. This does need proper accounting.
It is worth mentioning that, even if the electricity came from fossil-fuelled power stations, the emissions might still be lower overall because of the potentially-higher efficiencies of fuel usage in a large stationary engine than in a lot of smaller mobile engines working at variable load, spending part of the time idling and subject to less-rigorous maintenance schedules.
However, we need to keep proper accounts and not treat electric vehicles as if they were magically free of all environmental costs.