Twenty trucks is really nothing for a company like FedEx. As the article noted, this is a very small pilot project for them.
At 3 billion miles driven per year, there's a lot of room for experimenting with a few hundred/thousand new vehicles.
Anyway, electric freight works well for this business assuming the range is decent. There are lots of lower range freight routes with well forecasted volume, and controlled environments at both ends allows for installing charging infrastructure.
It’s a smart move. Between passenger vehicles, stationary storage, and their semis, the Gigafactory should be fully utilized.