What the study authors really want to say is that on top of the mountain of subsidies (i.e. taxpayer money forked out without their direct consent) that Electric and Hydrogen busses already get they need a load on top in order to reach cost parity with diesel for the operator. The total cost stack does not matter in such calculations as its always covered by the magical pond of taxpayer money. Money spent on hyper-expensive vehicles is not available anymore for other things such as public services.
Hydrogen fuel-cell buses still more expensive than diesel ones to operate, according to study by AC Transit