Even if it were cleanroom reverse-engineered, the code contains many constants relating to game balance (and many other things) that are identical to RCT2 or RC

Even if it were cleanroom reverse-engineered, the code contains many constants relating to game balance (and many other things) that are identical to RCT2 or RCTC (RollerCoaster Tycoon: Classic, the mobile port which was also released on PC), without which the game would not function, which probably makes it a derivative work. That being said, I'm fairly certain it was not cleanroom reverse-engineered. The not-so-legal status, then, is "Atari does not care".