Copyright violation is definitely different from plagerism. Changing the wording gets you off the copyright hook, but not the plagerism hook. Standards are a little different in editorial writing; stealing ideas is SOP. Academics are a bit twitchier.
Plagiarism, like copyright law, is generally exposed by the "parallel text" method used on this thread. It shades rather smoothly into other intellectual property areas, as you point out, but it's about words.
When I say what I think, I'm not claiming I invented all that. No one can keep a name tag on every idea.