Single axle shouldn't be an issue in most single trailer setups provided you're staying within weight limits in most circumstances, and you still have the full power of the ICE and its driven wheels if you have to start on a steep incline. Would have assumed the electrical motor(s) would be between the wheels and driving a short shaft to each driven wheel though (it's not specified in the article either way). Putting the engine anywhere else means you need a differential and it's going to need to be in the middle of where the battery pack is (generating heat).
Centralised motors as close to the wheels as practical is standard design for most EVs - shorter axless= reduces risk of breakage. 2 motors, one per wheel also reduces complexity and likelihood of failure (and increases HP available at the wheel) it's how I would design it (and most high HP EVs are designed)