Dumas was meticulously accurate, not to the world as it historically existed, but to how the French upper classes felt and wrote about. He was extremely well read in people's memoirs and diaries, and wrote his stories set in the world as the French aristocracy imagined it existed.

I believe he got this detail right in both ways; in that firearms were the most important weapons, and also the main characters would have done their very best to ignore that fact.

One thing that always bothered me was his use of currency. In the French original he mentions at least 5-6 types of currency and it seems they all have common sub-divisions, despite some of them being Spanish or even Italian.

Was France using other people's currrncy back then ?

For more on this, see the (sadly, hastily researched to some degree) biography of his father, _The Black Count_:

https://www.goodreads.com/book/show/13330922-the-black-count

which arguably served as a template for a famous novel which also features a count in the title.