Correct.

There is no cost involved, nor any license required, to develop a plugin for Acrobat. Just build the plugin, drop it in the Plugins directory, and it loads in Acrobat. No muss - no fuss!

This is the case because we wish to control what additional functionality is added to our free product. For example, we wouldn't want developers adding features of Acrobat to Reader - that would go against our product positioning. As such, plugins need a license for Reader.