Or rather, the description of the
historical tag is "The setting of a historical manga takes place at some point in Earth's past. The level of dedication to portraying the lifestyles, societies, and technologies of past periods and peoples accurately or believably can vary greatly between different works."—even though the tag descriptions on the site are from early in the site's history and do not necessarily reflect current usage for all tags, I see no reason this one should not be broadly correct. And this series seems to fit that bill?
(I was going to cite what happens if one searches for the historical tag, but honestly the results are abysmal XD There's some wuxia in there... I guess that might be "historical" in the sense that, say, something based on a King Arthur legend would be?! But anyway, I feel it's safe to say this is more firmly historically-founded than at least half of them. >_>; The fact that it doesn't wear it on its sleeve by giving any specific and real places and dates notwithstanding.)