I did not see any 'Big Bertha' images in the video, which included some footage of fortress and coast defense guns that were neither Belgian nor German.
I agree, Marc. The fat coast defence guns look like American 12in coastal mortars.
Meanwhile, in a set of DVDs called The Century of Warfare, there's a clip of a massive German howitzer being fired. A similar clip also appears in Mark Williams' Big Bangs, in which it appears to be the same firing event but from a slightly different camera angle, indication that at least two movie cameras were present.
The weapon looks to me most likely to be either the Krupp 30.5cm Beta IR or (my second choice) the Krupp 28cm L/14.