Technically he's right Rafe. Even though there were Americans there during the BoB, the Eagle Squadrons 71, 121 and 133 did not exist until the end of September 1940 (71 sq.). Even if you want to stretch the BoB to October, no operational sorties were flown by the squadron until February. They had to undergo familiarisation with the planes, be brought up to operational strength, and wait for standard rotation into front line service like every other squadron. USAF museum at WPAFB in Ohio says in their display of Eagle Squadron memorabilia that the first combat mission flown was on February 5th 1941 by 71 sq. Formation of 121 and 133 quickly followed as American volunteers at the end of 1940 and early 1941 were brought in and given assignments. Some people may consider that "BoB" refers to any fighting that occured from the time Germany attacked until the US 8th AF was on the ground in 1942. Maybe thats what your reference was saying.
