An agnostic is someone who believes we don't or can't know that a god exists. There is nothing about theism/atheism that precludes agnosticism. Each one can and does exist with it in many people.
Just pointing out that there are two valid definitions for "agnostic" in common usage.
You have stated what I call the "technical" definition. It is familiar to most RFers and philosophy majors.
But the word "agnostic" is also commonly used to refer to people who have no set opinion either way; they are undecided, usually because they think the arguments for and against are equally convincing or lacking.
It is a useful term, and I see no reason to pretend that it doesn't exist.