I've literally spent 2 years working on a project that did exactly what this article is recommending. There were some places which needed the relative-url as an identifier, and other places which needed the "database id" as the identifier. We constantly had to extract the id from the URL, or convert the id into a URL, and keep a mental map of which format each input was using, and which format was needed for each output. It was a mess. I would personally not recommended this at all.
If some parts of your API used database keys as identifiers and other parts used URLs, then I can see that could be confusing. All one or all the other would probably be better.
Anytime you're interacting with the database, you'll need to use database keys. Anytime you're interacting at the API interface level, you'll need URLs. Anything in the middle is then going to be a gray area, especially when you have multiple people working on this implementation together.
