These aren't all clones. Many of them are just playable versions of games that have had their source code released. IMO a clone refers to a new implementation, but a lot of these are just modified and re-released versions of the actual game.
Also some of them aren't clones at all as I understand the term. The "similar" tag covers this, I guess. "Playable" also seems to have a pretty nebulous meaning. The absence of "playable" certainly does not mean "not playable."
the term clone has been used that way a lot even commercially just pointing to game mechanics and other game elements. i do believe what this list is showing is probably more in line with the term clone though where it is basically a copy but their own assets or the original assets(you had to buy the game and thus have the assets) but their own engine which is less common but to name a couple openmw, and gemrb.
