Responses
It’s probably people just mis-tagging it. I see it all the time, especially with BL. I would say it’s safe to assume that this is Shounen Ai (no smut).
As for the “Adult” and “Mature” tags it could maybe have something to do with themes in the plot, but I would have to read more to know for sure. But, yes those are usually used to indicate other things related to Yaoi or smut.
I thought "Adult" and "Mature" tags mostly suggested smut or like heavy themes(such as abuse, etc.)? Maybe I'm wrong though. If they do suggest smut though, this is Shounen Ai only. There is no smut lol. It's still so good though, so I don't mind!