Arsenal is a silly word, unless you're a soccer fan. It has several different usages, and it has been used to describe any amount of weapons owned by an individual as well as massive government depots. So effectively the word is subjective and its appropriate use is determined by the user.
When the press used to think it's job was to report what happened, when and where and how .... they'd have said "Arsenal" if an industrial facility where arms and munitions were made and stored were the what or where in the story ..... now that they think there job is to "help the reader form pictures in his or her head and explain why this matters to him or her" ...... they are trying to paint a picture of huge numbers of guns and munitions .... sigh ....