A draft specification is not an official. It is just a draft which can be changed.
I can imagine UBUNTU adjusted all packages to the draft, however if you just download packages in source and compile them, result is a mixture of Icon=xxx.png and Icon=xxx
Key question is the adaptation of the draft by the DE developers. Maybe UBUNTU is strong enough to force application developers to follow it. Anyhow, if you use a certain DE you have to accept its implementation way.
Also, example is one thing. Detailed text:
"Icon to display in file manager, menus, etc. If the name is an absolute path, the given file will be used. If the name is not an absolute path, the algorithm described in the Icon Theme Specification will be used to locate the icon."
It still leaves room for different interpretations.
At the very end question is not what is written in the standard but what and how is implemented in KDE, Gnome, Xfce4, LXDE. You need to adjust your system, your applications to work as expected when used independently what is in the papers