They found one really long mail route that couldn't be serviced by an EV, so it's just a matter of crafting the requirements that every vehicle must handle every route, and you get the preordained outcome.
As the lawsuit says, they considered two plans: 10 percent EV and 100 percent EV. 100 percent EV rejected because there are unsuitable routes. Never considered 90 percent EV, or made any effort to identify what percentage would be feasible. The 100 percent plan was obviously designed to fail; just a fig leaf that they tried due diligence.