There are not only just POM files at https://plugins.gradle.org/m2. There are JAR files not available on JCenter as well.
Specifically, there are POM files that map the plugin ID to the actual artifact coordinates for the plugin. There’s no JAR at the location of the marker interface, but plugins published directly to the plugin repository then have a POM and JAR at the actual coordinates. Third party dependencies redirect to JCenter, but there’s also some plugins that only publish their JAR to JCenter and rely on this redirect logic as well. Effectively, the plugin portal is a superset of JCenter with additional POMs and JARs.