I think the idea is that you're powering traditional combustion engines with a fuel generated with green power. It may be more wasteful of that green electricity than a new EV charging from the grid would be but you didn't have to build and buy a new thing and it's kind of fine to waste renewable energy if the economics works out because it's renewable.

So it's effectively a way to convert your ICE car into a zero-emission EV by using that fuel.