In one sense, WebGL is a 4D library, since most vertex and color buffers are vec4s. In another sense, WebGL is 3D, since the vertices (and colors, if you're using premultiplied alpha: http://core.ac.uk/download/pdf/1578955.pdf) are in 3-dimensional projective space. In another sense, WebGL is a 2D library, since it's outputting 2D fragments.

The author is just making it clear that there's no magic here. It's all just 2D points on a screen eventually.