Perhaps I could try to respond to that.
We're not aware of any standard by which you could claim to be phosphate-free and the basis for those claims. I think what they're really referring to is that they do not use phosphorous-containing products as detergent builders. When you build a detergent there has to be a difference in what you're basing your cleaning power on, whether it's phosphate or synthetic detergents, etc. But technically, as a chemist I can say it's very, very difficult to guarantee that an element is not present. They're naturally occurring, and they can occur at trace levels.
I'm not aware of any actually scientific rationale one could use to say something was phosphate-free. It's an operational definition. I think what they're really getting at there is that they're not using phosphorus-containing products as the basis for their detergent.