The muffler pressure is very very low at low throttle settings though, so the engine needs to draw fuel on its own, essentially, at idle.

As the piston travels up and the crank inlet opens, a low pressure is created inside the crankcase. The pressure outside the crankcase is higher (and inside the fuel tank) which then the fuel is pushed into the carb. At high throttle settings the muffler pressure provides an extra boost pushing fuel into the engine albeit not much.