The statement was "terrain is terrain". But the tiles are not terrain, but more than a game board. Just one that happens to be modular.
Take one of those people that made Drow Outpost, or some of those other maps in 3D and there you have some terrain.
2D isn't terrain to a player. The green blob may suppose to be a bush or tree, but you have to ask. Terrain would be something that tells you right away even if it is a Lego tree. It needs depth beyond the tiles flat images to be terrain.
The tiles are good visual aids, but not quite terrain. Those fold-up models WotC has archived somewhere are good examples of terrain.
Now, if you have a pull-out tap like a pop-up book on the tiles to pop up the trees or something I will call them more akin to terrain.
Otherwise, they are just a modular map that indicate what terrain
would exist in 3D just like a surveyors map, but without the contour lines.