It’s not necessarily an issue of fuel, but the overall wear on the components and engine when you start a car. A starter motor only has so many “starts” it can do before dying. The battery too.
Starter motors have gotten a lot better since the “bad old days” and engines start more smoothly thanks to fuel injection and computer control systems, so manufacturers have decided that it’s ok to start/stop engines as needed, but the reason for not doing it was never a matter of fuel savings.