That is suprising. Ive got Iron steam pipes in almost direct contact with wood where they pass through the floors and 80+ years of exposure to 215F iron hasn't caused any charring. But to address the overall point - I agree that clearances are not something even worth questioning. Just dont do it. Sure the clearances might be overkill ofr daily operation - but that's what a factor of safety is all about... In fact I might guess, and hope that the clearances are set to protect in case of an accidental overfire, not just daily proper operation.