I don't recall Cameron saying or admitting to stealing from Ellison but Ellison has had a great many lawsuits in the past. He's quick to sue and you can't deny that. Whether or not all those suits were because people stole his idea directly is another matter I suppose.
Anyhow back to the original claim, I agree the very simple 7 stories remark is vague but it is true. It's whether someone stole the framework used from another person that matters and again, this idea is not new either. If you looked hard enough you can probably find it predating Poul Anderson too.