It would make the use of a smaller displacement diesel engine practical, and fuel tanks could be smaller for comparable range, so some or all of the weight from the battery pack might be offset. Still, since this will be a bespoke design, they'll be looking for weight savings wherever practical and cost effective.. The article referred to "dual mode," so a combination of direct drive and a "reservoir" of recaptured waste energy is what they're talking about, electric assist from reserves in the battery pack for initial acceleration and hills, via computerized sensors of some kind.
"It would make the use of a smaller displacement diesel engine practical, and fuel tanks could be smaller for comparable range"
Ok, here's a little analysis after a bit of research.
Typical max weight for a tractor/trailer is 80,000lbs
Horsepower for a topline Peterbilt is 600hp
Let's assume that to be effective the engine will have to be at least 100hp smaller, representing about a 17% increase in efficiency.
Power density of a Lithium Ion battery is about 160Wh/kg
I'm assuming that accounting for power conversion inefficiency, a 50KWh battery plant will be necessary to accomodate the smaller horsepower - this is enough power to compensate for the 100 HP smaller motor, and keep the Lithium Ion batteries from dipping below the same 70-80% charge rate, that hybrid cars are designed for for max longevity.
This will allow the plant to support 100hp of output for 6 minutes.
This is about 750 lbs of batteries - add packaging, wiring, etc, call it an even 1000 lbs. Add the motors, transmission and other components, say another 1000 lbs.
Add the fact that you'll need to recharge the batteries after they are used, you are reducing payload by 2.5%
all in all the 17% figure is probably gonna be halved - call it 8%.
So for 2000lbs of lower payload, you get a potential 8% increase in efficiency. That may not allow tanks to be too much smaller, but it's nothing to sneeze at over a large fleet of vehicles.
Ok admittedly, it's back-of-the-envelope rough calcs but I can see how this might work. The key is that the additional complexity is reliable enough to not increase acquisition or maintenance costs too much.