I think I've read that 'caliber' is formally defined as 'hundredths of an inch', as in "45 caliber". If so, then "84mm caliber" would indeed be confusing (and incorrect ;^)...
In naval artillery parlance caliber is barrel length measured by the multiple of the projectile diameter. For example the USS New Jersey has 16 inch, 50 caliber cannons, meaning the barrels are 50X16 inches.
But that would make sense in this article.