Yes, it is very confusing and "somebody" should definitely change such confusing parts of docs, as a general rule typical "moodle docs" are written for moodle 1.X.X and moodle 2 has separate moodle 2 docs for moodle 2.X.X
If you see download links like
Download for Moodle 2.0
you can safely use these plugins with moodle 2 but not with moodle 1.9.
Or if you see in Modules and plugins requirements tab text like
Requires: Moodle 2.0 or later
you can use those plugins with moodle 2 but not with moodle 1.9.
In non-core plugins REQUIREMENTS - Moodle 1.9 or later, 1.9 or later does not usually mean that those plugins automatically work with moodle 2.X, usually they don't ... or have not been tested and upgraded to work with moodle 2.X.
All core plugins are using code written for each core version and what ever non-core plugins you test you should install and use them at your own risk