It's a neat story, but the claim to "biggest on Earth" is hard to substantiate. Other articles just tend to refer to it as "one of the largest". The real issue is in how one measures "largest". The entire complex covered an area 9 miles by 2 miles per a couple of different sources. However, it was not all one building, but a series of spread out smaller buildings. This was done so that an accident in one area would not damage other buildings. The entire thing was also made to be taken down after the war, so it was builkt very light as the manufacturing of cordite was a relatively "light" manufacturing process. If mesuring by "production floor square footage" the facility was smaller than places like the Packard Factory in Detroit.
A similar facility to this one existed in South Amboy New Jersey at the same time and was called the Morgan Depot/T.A. Gillespie Shell Loading Plant. This one was also billed as the "largest in the world". Unfortunately it exploded in October 1918. The result was the displacement of over 62,000 people who had to be evacuated from the plant and the destruction of large chunks of both South Amboy and Sayreville New Jersey. They are still occasionally finding ordnance from that blast buried around that area.
T. A. Gillespie Company Shell Loading Plant explosion - Wikipedia, the free encyclopedia
GREAT MUNITION PLANT BLOWN UP - 100 MAY BE DEAD - Series of Explosions Wrecks the Gillespie Shell-Loading Works at South Amboy. LARGEST IN THE WORLD Survivors Say That of 2,000 Men on Night Shift Hundreds Are Dead or Wounded. MANY TONS OF TNT SET OFF