I think a lot depends on the effect you're going for.
I'm reading Red Grass River. It's set in S Florida c 1910-30, and is a realistic tale of bootleggers and bank robbers. There's also a sub-plot about second sight, and revelations through dreams. So is James Carlos Blake's hard-boiled historical crime saga really a historical fantasy that should be shelved with books about dragons and fairies!?
Not hardly.
I suppose you could. But that would raise more questions about your critical judgement than Blake's.
I didn't read The Princess Bride and get annoyed because Florin & Guilder are coinage rather than Medieval political entities. Goldman is trying to evoke the feel of a fairy-tale, not a historical era. He succeeds, quite well.
Another example, in Walker (Alex Cox's film about William Walker's invasion of Nicaragua in 1856), when Vanderbilt hears Walker has double crossed him, he kicks over the TV. When Walker's mercenaries flee the burning city of Rivas, a helicopter lands and a US Marine with an M-60 asks for their passports. I didn't think, "Geez, Walker should have used the helicopters and machine guns to whomp those bass-ackwards 1850s campesinos." I could see Cox was (in a ham-fisted fashion) using anachronisms to draw attention to the parallels of the 1850s and 1980s.
There are lots and lots of examples of anachronisms in fiction which you could deconstruct & criticize endlessly (it's fun). That'll show you what some other story-teller was doing.
So maybe you should ask yourself, what are you trying to achieve with your time-displaced Merovingians? Are they there because they are cool, or to make a dramatic contrast, or create a certain mood and feel?