Whatever a combo box could do, the suggested use is to select a tag, as that is what is written there. Selecting and guessing a tag are two different things to my understanding.
Not even the fact that I can type in that box would hint at tags existing, that are not in the list. It just says filter with. And it does filter the list if you type parts of tags, like orr for horrible and horror.
It goes even further. There is a link to the right below the box that says view all tags. And there is a button on that page that says find tag.
Why does that link not read featured tags instead of all? Also the find button does not find tags. Even if I put banana in it, it will find nothing. It is just the same combo box for the featured tags but with find written in it, instead of select.
That functionality is not poorly advertised, as I wrote. It is implied that is does not exist. I found out by accident, after deducing that it must be possible somehow, because I encountered tags in game descritptions that I could not select in browse.
To go back to topic, hexagon and match-3 are tags that have to be put in exactly to find the game in browse, and they have to be typed in the tag box. The match-3 surprised me. But the game can be found if players click the match-3 tag on one of the other 300+ games that have tagged this. And I guess it helps for related games or recommendations.