"Based on" meaning that's what they started with: the same heavy delivery truck that everyone uses. It's clearly not a Transit (USPS already has E Transits in service) but they obviously didn't just take a light truck and then add a pound to skirt emissions, either.
Edit: looks like it was an early prototype from Oshkosh with very little in common with the final product:
https://www.caranddriver.com/news/a19445176/this-ford-transi...
and
https://www.freep.com/story/money/cars/ford/2021/02/23/oshko... mentions not using the Transit design.