And as it happens, I tested this out with webfonts. The results I got on OS X look a whole lot like the image on the article; it turns out that e.g. SourceSansPro-Light.ttf vs. SourceSansPro-LightIt.ttf just have a different weight, which is also apparent in the fact that the glyphs are clearly smaller resulting in more text fitting on screen in the italic version. (Stupid me. Had I thought about this a little more, I wouldn't have realized that this proves that the glyphs aren't same size and therefore also not the same weight.)
On the other hand, if you just tell browser to do "font-style: italic" on the ExtraLight ttf file, the glyph widths remain the same, and the font weight is probably almost the same. If italic is generated from the regular shape by shearing the glyph control points, then the shearing also distorts the weights a bit, but it doesn't bother me. In fact, I can't notice it by eye.