The quote you're referring to came from the study's lead author, a Brit. Presumably he was referring to soccer (which everyone else calls "football"). Furthermore, a soccer field and an American football field are reasonably close in area, so it gets the point across in any case.
However, I will now refer to explosives exclusively in units of Bald Eagles when referring to their mass, and "star spangles" when referring to their energy output.
By analogy with another type of football, i.e. rugby football which was nicknamed rugger in the old days. Soccer was always a nickname, and association football the formal name.
The term "football" was used in medieval England to refer to sports played on foot as opposed to on horseback. There was a class element too as football games (of which there were many types) were played by the common man while equestrian sports were the realm of the elites. There were even laws passed regulating how much "football" could be played so it wouldn't distract from other peasant responsibilities.
However, I will now refer to explosives exclusively in units of Bald Eagles when referring to their mass, and "star spangles" when referring to their energy output.