It's made in RPG Maker 2003, cause it was made for a game jam that stipulates it! (Still open for about a week, at the time of writing)
As far as I know, EasyRPG has both an editor and a player, and the editor isn't really ready for use yet, but the player is, and since it has a web version, you can use it to play RM2k3 games in the browser. Lots of people have been doing that for the jam.