What he means is, if you want to download the document from ISO that describes the standard, you have to pay a fee. Here’s their store page: click.
It’s about 190 USD for a 38 page document describing the rules of the standard. There’s another document with extensions for a similar price. Quite pricey for a PDF file obviously, and the RFC is free to download.
On the other hand, no one in the history of time has gone “hmm, I don’t know how ISO-8601 works, let me go buy this document from the ISO store to figure it out.” Most people just call datetime.isoformat() or whatever their library function is called.