I think there is some truth to it. Focusing heavily on conditioning will diminish your strength potential. I believe in a sport like boxing, conditioning, specifically anaerobic conditioning is more important than strength. A lot of punching power is produced from proper technique rather than just raw explosive strength. You want both, but somebody focusing their training on power lifting and strength gains or strong man competitions will have noticeably more strength potential and lifting power than somebody who splits between power lifting routines, calisthenics, aerobic/anaerobic conditioning, sport specific boxing drills/technique, etc.