I dont believe this at all. A carb alone can not increase fuel mileage a great deal. Internal combustion engines (ICE) need a specific amount of fuel to make a specific amount of power, to get the claimed 50 MPG from just simply a carb the fuel would need higher BTU's. Electronic feedback fuel injection does a really good job of maintaining maximum economical air/fuel ratios and the only cars coming close to 50 MPG are very small, light, and under powered cars. If they cant get 50 MPG with EFI it certainly will not happen with a carburetor in which air/fuel ratio is controlled by venturis, orifices, needles, jets, and variable airflow.
There's a million of these kind of Grandpa stories out there. They invented a tire that lasts forever, but you'll never see it, because too many industries would suffer, or something to that effect.