It's just that finding the "correct" ones may become hard as soon as forks of the same plugin can be found there, or plugins that coincidentally share the same name (because they are not checked for uniqueness, unlike in the plugin directory).
The package names are in fact unique also in Packagist, so it isn't possible to accidentally install a wrong one.
But I agree it would be nice to have some kind of direct relation between them. For example the name of the Composer package (if one exists) could be mentioned in Moodle's plugin repository.
For anyone not familiar on finding the Composer package name of a Moodle plugin, here are the steps for it:
1. Find the plugin at Moodle's plugin repository
2. Find the "Source control URL" link from the plugin page
3. In the source control repository select the "composer.json" file
4. You can now read the package name from the file
For example in this case the name is "fmcorz/moodle-block_xp", so the command for installing the plugin would be:
composer require fmcorz/moodle-block_xp