Excuse me if this was already obvious, but the word "core" designates a new product line, starting from 1.0. Obviously, it's informed heavily from the old .net framework, but it's not backwards compatible with it.
I could see why others might see .Net Core as a parallel product as opposed to a new one.
Overall I think it's a good move just a little confusing (unless I'm the only one) for those who have done .Net work before.