I agree that there was nothing else to link to, especially at the time. It was an understandable choice. But it comes with a cost: until that tutorial gets rewritten to point to a more appropriate specification or explainer, new readers will come across PEP 517 expecting it to be user documentation. They will notice its shortcomings in that role. It should not be surprising.
Fortunately, another discussion thread, Removing setup.cfg and setup.py from the packaging tutorial - #91 by craigf, has also suggested that more suitable user documentation should replace the references to PEP 517 and PEP 518 in the tutorial.