I am not sure why RHC issuing a SN should be treated with suspicion? For RHC to change the required pages in the POH, which have to be FAA approved takes time. Issuing information via a SN does not. Sounds like "damned if you do, damned if you don't" RHC are stating in a publication that the pilot can take into account the reduction in engine output power when using carb' heat. Not sure where the "legally unusable" come from.
I am not sure what is meant by over boosting through the gearbox. Over boosting is usually something the pilot might do to a turbocharged or supercharged engine (you cannot over boost a normally aspirated engine as in the Robinson R22, R44) Exceeding the BHP limits will increase the drive train load per cycle thus subjecting the components to greater loads than RHC applied during their fatigue testing - the pilot becomes a test pilot.
Full throttle on your engine is a physical stop, after this point the engine will not produce any more HP.
We are not talking about the risk of exceeding the limited BHP but getting across to the pilot the effect of the application of carb' heat on the power output of the engine.