Now is this a different use of the word "Core" that we now (two years later) think it should have been used as? Sure. I'll buy that. But we can't put a definition onto the word that it wasn't meant to have.
Except Core had a definition well known to their customers before this. In 3rd Edition days "Core" were the three books every DM was expected to have: Player's Guide, DM's Guide, and Monster Manual. People felt it was a mistake two years ago to call everything "Core", that doesn't make it hindsight now.
I understand the reasons they wanted to call everything core, but the fact that people have been calling them out on it since day 1 is fairly good evidence that it was a mistake to do so.