All true. And yet…
The fossil engine can run at one speed, maximizing its efficiency, which is a non-trivial offset to the efficiency losses. Still, they’re there. Of course, if you wanted to have STOL with fossil engines, you could put a bunch of them out along the wing… but we all know that’s a non-starter (ouch). Relative to doing that, doing this is vastly more reliable and probably much more economically viable. So, they’re giving it a shot, to see how it works out.