Cars have a wide operating band, which is why hybrids make sense. Fundamentally, hybrid cars want to run the engine at a narrow set of operating points that are most efficient, and use a reservoir to do time arbitrage on the energy generated during driving. It's more than just regen to recover energy already spent -- the motor also lets you navigate the engine map to pick your point of operation. Ideally, the engine and the wheels are fully decoupled, at which point the engine rpm would just be constant. The battery and motor just move energy around.

Planes aren't really like this, they're actually a lot closer to boats. They move from fixed operating point to fixed operating point, and the engine is optimized around this. There's a lot less fat to trim, because the engines already have a narrow operating band.

That's not to say that this won't work; Pratt obviously knows what they're doing. But it's probably not an accident they've gone for smaller turboprops. I actually think the biggest opportunity for hybrid propulsion is in smaller drones where you want high performance piston engines, because tip clearance becomes an issue for gas turbines. A turboprop might suffer similar performance challenges that would make a hybrid configuration make sense.

Edit: just saw a post below on the relevant patents. Looks like they're using hybridization to decouple the two shafts. Without the motors, they're coupled via the gas path. With the motors, you can navigate the maps of the two shafts independently. I'm guessing this lets them pick a combination of shaft states that saves fuel. Most of that is pretty uncontroversial, but I'm surprised the claimed fuel savings are that high. As I mentioned above, it's probably because it's a turboprop on a small aircraft.