Its gameplay mechanisms are pure 2D, though. Simcity 2000 and up are regarded 2.5D by some. Sure, a change in terrain height can mean higher land values, but you could emulate that in 2D as well.
I hold to my definition that if the 3rd dimension affects gameplay, it is counted as half a dimension if the rendering technique used is not full polygonal 3D.
Though it definitely is connected to the visualisation you choose to use.
Sim City 2k graphics is 2D, and the "emulation" of depth is achieved by isometric techniques.
If you would add terrain height change to any 2D game, you would need some way to show that the terrain height change had happened.
If you do this with some graphical effect, like some shading, a scaling, fog, etc, that give some illusion of depth, the game would then become a 2.5D game.
If you just have a number "5m above ocean" on every tile, then it remains a 2D game.
I must say though, that I'm a bit confused about the "3D games with 2D gameplay" though, that is, full polygonal 3D, but gameplay limited to 2D as the OP describes.
I'm not sure I would like to call that 2.5D, but I'm also not sure what I would want to call it probably something contrived like "2D game with 3D graphics"...