I've been tracking these guys on youtube for a couple of years now. Their engineering (and approach in general) is sound.
There are a number of advantages to a 'hybrid' approach, particularly for trucks that'll be running on logging roads. Electric drive means you're always in the right gear. A diesel generator means you're not tied to recharging facilities. As a side note, they checked "multiple times" with the BC provincial government (where they are) and as their fuel runs a generator which could easily be part of a stationary recharge facility they don't have to buy the fuel specifically for motor vehicles, which is taxed differently (I'm pretty sure BC is like most (all?) states in the US where carriers (semis) have to pay a road tax based on miles traveled as well). And as it's a hybrid, they get some additional tax benefits (programs designed to encourage non-ICE vehicles). Also, as a hybrid that goes up and down hills they can use regenerative braking to recover at least some of the energy used to either get up to speed (logging trucks IMO go terrifyingly fast on logging roads - but that's speed for conditions*) and go up hills. In both cases, traditional ICE vehicles throw away that potential energy as heat. And, as you mention, the engine driving the generator can stay within it's most efficient RPM range. Speaking of which, they even figured a way to use the generator as a motor and so (once batteries are full) use the engine's compression as a 'jake brake'. Color me impressed on that one. There's also the advantage of not needing ginormous batteries, so less lithium required per vechicle than full-electric - and diesel is much lighter per KWh.
Bear in mind that much of this development was done in plastic-sheeted Quonset huts.

*logging truck drivers famously collect on their life insurance at a fairly high rate - to the extent that one of the guys at Edison has proposed that if people want to chip in and buy him insurance, he'd be willing to assign those folks as beneficiaries of the policy. No idea if that's legally possible, but it does underline how dangerous those jobs are.