From a quick skim through the text, it seems to define what an immobiliser is, how it should work, and how it can be advertised on a car.

I don't see anything in there about them being mandatory across the EU. I know some member states passed laws mandating them before that document was published. Perhaps I got the wrong document.

Also take a look at 74/61/EEC. Some form of “immobilizer” has been required in Europe since 1998, and between actual ECE/UN directives and insurance partnerships, the standard increases every few years.