hey @rileydog Good question.
I believe that Plone IS a Content Management System. I think it's hard to say it's not.
My previous post
was a poor and immature attempt to shine a light on what I think is a bad conversation and a bad direction - arguing "what a CMS is and who is/is not in this category - because WE surely are!" I was a forum troll in a past life.
What's more important - and seems to be better suited for the "Repositioning Plone" thread Repositioning Plone - #3 by pigeonflight is to remove ourselves from preconceived categories, especially super ambiguous and downright dogmatic categories (like CMS) and really see what the value of Plone is. Once we begin saying that we compete in a CMS space, we can fall victim to Wordpress, Drupal, Django, and Wix that have more power to define what the requirements are to be a good CMS.
Now we are in the following instead of leading position.
Don't get me wrong, I want Wordpress ease for add-ons, the user interface of Wix or Squarespace, and the Security of Plone. But I'm not going to chase the word "CMS". I want to define it.