Wide open throttle and percentage of maximum output are two different concepts. An engine operating at partial throttle has higher pumping losses (pulling air past the partially closed throttle plates).

An engine operating at WOT but still producing a fraction of its maximum output is certainly possible (and generally more efficient than an engine producing the same output at a partial throttle setting). A diesel engine is almost always in this configuration (metering fuel but generally not restricting air with a throttle plate) and a gas engine at low RPM but WOT is also in this state.

If tuned for efficiency in that config, it can be very efficient and is how many piston airplane engines are regularly operated where fuel consumption is an important part of range (and operating economics).