The MPG is still based on total miles driven. They'll have to put a LOT more miles on their high MPG vehicles to counterbalance poor performers. That costs money. They'll look at dumping those poor performers. It isn't misleading. It is still calculated total miles driven (of all vehicles) / gasoline consumption = MPG.
There are no "credits" mentioned by the NHTSA so I have no idea where you are getting that idea from.
Also, included, automakers are REQUIRED to have every light duty vehicle have at least a 49 MPG average by 2026. That isn't even corporate fleets, that is every single vehicle made in that category.