It's tagged wrong, wouldn't be the first or last time on this site. Shounen Ai means a romantic relationship between guys, which is not the case in this story. Mangaupdates has the correct tags, since the tags are put there by the people who actually translate the story and so know what the mangaka tagged the story with.
All this story has is BL undertones as fanservice; not actual Shounen Ai. It's a tease; a trap, which is something Managupdates mentions.
If you're reading this hoping they'll end up together, you're going to have a bad time.
Also, by tagging it as Shounen Ai you are not warning people who are not simply warning people who aren't into the nonexistent Shounen ai in this story, but you are effectively making people hope for something that will never happen and just leave them disappointed. It should be tagged as it was intended.
Actually that's not always the case that the people who translate it tags in Mangaupdates, in fact not always put the info at all. It can be anyone who has account there.
Since last time it was a case that they credit the wrong author and readers that knows was angry at translators group, but they said to have credit the right authors name elsewhere and did not put data in Mangaupdates, (didn't know who did it)
They usually tag Shounen Ai along with shoujo is to warn readers that there are boys x boys activities. Especially by Anri who kisses both men as women!
So homophobes who came just to read action might know to expect that there's some scenes of gayness so they can skip this story (≧∀≦)
And it's also a bait for fujoshi.
But the correct tags are:
Genre: Historical, Action
Demographic: Shoujo
I'm suprised a lot of people still think this is shounen-ai. The tag is wrong, has been for a while. This story just has BL undertones, but there's no actual BL. It's essentially a trap.