It has been a long time since I last submitted anything to an anthology but in those days they used to prefer stories that came in at around 5000 words. That way they could include more stories and authors than if the submissions they were getting were longer. I recall remarks made about the cost of paper and ink, but I suspect this had more to do with the amount of money they had budgeted for paying the authors for the stories. Mind you, these were all markets that paid professional rates. (However, since some of the stories they accepted were under the requested length, they didn't mind if a few of us went over—as I seem to recall that I generally did—since the amount they were paying for stories averaged out to be the same and they didn't blow their budgets.)
These days, of course, a lot of anthologies are digital only, so in terms of producing a book, length is not so much an issue. But the fact that the anthology you are asking about doesn't say anything about an acceptable length for stories in their guidelines makes me wonder if it is a paying market, and if it is, do they pay by the word or a flat rate for stories regardless of length? A digital anthology that pays $25 per story of whatever length (or doesn't pay at all), is going to care a lot less about the length of the pieces they accept, than an anthology that plans on print and digital editions and pays writers 6 or 7 cents per word.
