A Midsummer Tempest XII

The only premise of Poul Anderson's A Midsummer Tempest is that Shakespeare's plays were not fictions but true histories. Why then does an Alexander Dumas character, the Musketeer d'Artagnan, suddenly appear on page 126? The seventeenth century is the authentic period for d'Artagnan but is Anderson extending his premise by also including a French novelist's works?

Alan Moore's The League of Extraordinary Gentlemen, which features  Shakespeare's Prospero, would certainly, if its action had moved into seventeenth century France, have encompassed d'Artagnan because the premise of that series is the co-existence of every possible fictitious character in a single history. But, so far, A Midsummer Tempest, has confined itself only to the consequences of Shakespeare's plays being histories. If d'Artagnan can cameo, then who else might be waiting in the wings?

However, a quick google search reveals that there was a historical d'Artagnan about whom at least two novelists, one of them Dumas, wrote fictionalised accounts. Indeed, Dumas' source for the character was an earlier novel. Thus, Anderson, no doubt knowing that there was a real d'Artagnan, inserts that historical character into his narrative, thus deceiving his audience into thinking that they are now reading about the hero of The Three Musketeers.

Still rereading A Midsummer Tempest, I still find its subtleties unpredictable and apparently inexhaustible.