The article you linked says:I read the wiki, but I can't find anything to source it's claims. We have this article from the St. Louis Fed which shows it at over 27 million tons in 1913 and 9 million in 1920, so that directly contradicts the Wiki. Also another citation about the situation in general and backing up the Fed article:
Which kinda backs up what I earlier quoted (because they say the production in coal in Pas de Nord (in which the Bethune mines are) dropped becaus of destruction of the mines):Comparing the situation in 1920 with that in 1913, it is evident that a good deal of progress has been made in developing coalmines in the south of France, in order to compen-sate in some measure for the destruction of the mines in the Nord and Pas de Calais district. Production during 1920 was, however, 41 percent smaller than in 1913.
(principle source: http://www.archivesnationales.culture.gouv.fr/camt/fr/egf/donnees_efg/1994_026/2010_028_INV.pdf )Pendant la guerre, la production baissa beaucoup, descendant même jusqu’à 332 000 tonnes en 1915-1916 (2 423 000 tonnes en 1913). L’extraction moyenne journalière qui était de 8 000 tonnes en 1913 tombe à 1 300 en décembre 1914, puis 1 000 en 1916. Ce ne futqu’en 1924 que les 11 sièges d’extraction purent à nouveau fonctionner pleinement.
With Google translate:
During the war, production dropped considerably, even falling to 332,000 tons in 1915-1916 (2,423,000 tons in 1913). The average daily extraction of 8,000 tons in 1913 fell to 1,300 in December 1914 and 1,000 in 1916. It was not until 1924 that the 11 extraction seats were able to function fully again.
The first two lines of the image you posted state that coal production in France dropped from 40 millions tons in 1913 to 20 million tons in 1914. And then describes that was due to german destruction. So indeed all evidence points to a severe drop of production of the Bethune coal mines during the war. Which is not surprising at all.