I believe usually "3D" implies that some change of perspective is possible, e.g. by changing the orientation of the camera.

The libraries that generate(d) such graphics typically only could render such scenes because limiting all edges to the three major axes and discarding perspective (distances on the projection do not get smaller the further the line is from the viewer in the 3D scene) makes it computationally a lot easier to render scenes.

To distinguish them from “true 3D” libraries, a name had to be found. 2.5D seemed fitting.

It seems to be a 3D world, projected onto a 2D screen, just like almost every other '3D' game. I think they're nit-picking because they think of isometric projections as being less "3D" than perspective ones.