I know! I totally agree. When I first came across the concept of plagiarism as replication of an idea, not just copyright theft word for word, I was surprised, because to me, that wasn't plagiarism. But I've seen been presented with Medium's definition and seen it elsewhere.

That obviously brings us to this discussion point - I knew nothing when I came into the world, and everything I know has been learnt from someone else, so everything is plagiarised.

At the same time, with art, it's copyright infringement that's the problem, and that's a bit different, because it's a straight copy, not just using an idea.