I want to point out that while the method in #9 would work, it doesn't mean that the method in README is incorrect. The method in #9 depends on adding a custom repository for the library which is not necessary (as that library is already available on Packagist). However, the package is of type library which causes it to be placed in the vendor directory. This was discussed extensively in #2918901: Fix documentation for composer library setup. There is also a PR to update the composer package type so that we could place it without using the package name in the custom installers path.
Between the two methods, I would prefer the method where we can use the package info from packagist rather than us specifying it in the repositories section. The simple reason is that updates are available without changing anything in composer.json file. Another reason is that this method involves changing very few lines compared to adding an entire repository section. Still, both methods work and someone who is well-versed with composer could choose to use the method which overrides repositories if that is what they need.