In my experience, the board filter for Join Games doesn't list a board if there are no games on that board available to join. 

However, just now I used the board filter to look for games on the Antastic board, which was listed in the board filter, but when I selected the Ant board from the list the site said no games were available to join on that board.

Are there hidden games that can't be joined by everyone, or without certain criteria?

It appears on the board filter because there is an open game on Antastic.

No games available for YOU to join, because you are the one who created the only open game.

i.e. Board filter is populated the same for everyone. If there is an open game, it is there in the board filter. But despite it being there in the board filter, a particular player may not have anything available to join if they created the only open games, or if all open games are created by someone who has them on their enemy list (and that person is non-premium).

Ah, I see.  Thank you.