If a medical doctors classifies himself as world class and then posts articles about medical matters, then my expectation would be his world class-ness is evident.

This guy's not writing an opinion piece about which brand ketchup tastes best. He has published an opinion piece in which he speaks authoritatively about matters related to the domain he claims he's a world expert in.

As for the harmless exaggeration, it's not harmless. It's poison to my chosen profession.