This is a very low power output.
Efficiencies are rated for full load, where the IC supply and biasing requirements are negligible. Plus the Mosfet drive current, which at high frequencies can be significant.
For this purpose, there are many controllers with features like "pulse-skipping" and "frequency foldback" options, which dynamically adjust their operating conditions for low output current operation.
Linear, TI, Onsemi, STMicro and others offer such regulators.