tags are not the nsfw indicator. The game is properly marked. You actually cannot see the nsfw status on any nsfw game's page.
There are even sfw games that do have the tag "adult". Tags are not meta information. Look here for example https://xformgames.itch.io/gladiator-true-story Paid game, indexed, with "adult" tag. But it is not nsfw under the criteria of an US based website. But nude pixel art is. It's not an age rating, and yes, that is confusing.
And that taffy dev is an old and popular developer with 8k followers. Why do you think their Chapter 2 is top seller almost immediatly after publishing.
And as I said elsewhere, there is no strict rule that says that paid nsfw games are not allowed or impossible to be indexed. It is just the way most games are treated.
(The relationship between the "adult" tags and the nsfw status is this: if you select one of the "adult" tags, you get a firm reminder about maybe marking your game as sensitive content.)