Three quotes to answer the question:

The Mosquito is a dispensated class under the F16 rules. It is actually too light to fit the F16 rules but because the rig size and platform width are well under the maximums allowed, the Mosquito is accepted in Open F16 Events (Tim Shepperd).

The Mosquito is part of the growing F16 movement and is dispensated for being underweight according to the F16 box rule (Mosquito flyer).

The F16 association also has an Open F16 division, which allows grandfathered and dispensated f16s such as the taipan 4.9+spin and the Mosquito+spin to race in the F16 competition, but with their own boat specific yardsticks (80 for the mossie+kite for example) (MCGriffith).