Try a three minute round of sparring and you will soon change your mind on that. FWIW a study found gymnasts to be the all round fittest athletes.
This is from Scott Quigg's wiki about his fitness being tested at the University of Salford.
'Bury's Quigg (24-0-1) spent a day with his strength and conditioning coach Ric Moylan University of Salford, Human Performance Laboratory. The results left professionals in that field amazed as the British champ flew through the assessments. Moylan revealed: "When it came to fitness levels, Scott was up there with elite endurance athletes like Sir Chris Hoy and Mo Farah. I have been involved with professional sportsmen for 15 years and Scott is one of the fittest guys I have ever seen. He did fitness, performance and endurance tests, blood analysis, and urine tests, but Scott's determination was equally as impressive as his world class fitness.'