Yea, that wasn't super clear to me, either. But... given that turbine APUs already exist for ground power and are already engineered to be as light as practica

Yea, that wasn't super clear to me, either. But... given that turbine APUs already exist for ground power and are already engineered to be as light as practical, my naive guess would be that it's easier, simpler and faster to just use a COTS APU as a series hybrid rather than trying directly couple a turbine to the propeller. On inspections and overhauls: I'd expect that not needing to start the turbine at all on a given flight massively helps the economics of the airplane. Turbines often have parts that must be replaced after a given number of (startup) cycles, and that's going to be especially important for a turbine that is only expected to run for a short time (if at all) at the end of a flight. If you only start the turbine on, say, 5% of flights, that means you get 20x as many flights before overhaul/inspection. I could see that being incredibly attractive to airlines.