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 think we're on the same page about this for sure! I looked through the mangaupdates site and this is definitely classified as shounen ai. The genre list also doesn't reflect that there are any heavy themes in this story, just romance, comedy, etc. So I do believe you're correct that it's most likely been tagged incorrectly.
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!