For me, when you get to the point where you can construct two valid statements that both are supported by available information, yet one disproves a statement and the other proves it, you have reached the point of 'nits'.
In this case :
"They are not zero emission because electricity can come from a coal fired plant."
"The are zero emission because electricity can come from a nuclear power plant."
Pedants could go a different way, they could say "the bus is zero emission but the infrastructure isn't."
We saw a lot of that with solar panels where people would argue that the energy to smelt the aluminum to make the frames and furnaces to grow the silicon ingots far exceeded any amount of energy that the solar cells themselves would provide.
But one has to wonder, what is the point of arguing at that level when, as the article states, the pollution where the buses are deployed is significantly less?
Pollution where deployed isn't the issue, overall pollution is (the main impact of CO2 is climate change on a global scale). Significant reductions in pollution are to be celebrated, but it's wrong - and takes away from your point - to emphasise something as "zero emission" when it isn't.
I think inhabitants of large Chinese cities that have to wear masks when they go for a walk would disagree.
Actually, both things are quite relevant. Pollution in cities poisons and kills people in the short run, climate change would kill us all in the long run.