Well, the etymology of month is moon, which is why even in the real world the month is about as long as a full set of phases. Of course, it doesn't quite line up exactly to a nice round number with our pesky habit of orbiting the Sun in 365.24 days, so we've fudged it a bit to make our calendar match the seasons instead of strictly following the moon (a lunisolar calendar). In the game they have no such restrictions and can artificially make them line up perfectly, which is easier to code and write lore for.

I don't think there's anything wrong in calling it a month, but yes they do refer to it in game as a moon. And they just all happen to have 32 days per month with 4 weeks of 8 days each, exactly. Because why not. Other than that it's really not that different from our real calendar at all.