smut has more to do with the intention of the work, and less to do with the content. this work exists primarily to be a depiction of something sexually stimulating, any story or plot is purely secondary. that's probably why it has the tag. you'll notice it doesn't have an h tag, because it's not particularly h. they're not necessarily the same thing, and smut is not simply an escalation of h.
for what it's worth i don't care if it has the tag or not. this is just my take on why it's there.