Notice he says that it wouldn't be fully open source.

Of course chimeracoder noticed this. That's what he was talking about. Because "open-source" is a binary condition, being "not fully open-source" is like being "not fully pregnant" — the phrase is just an equivocation that ultimately means the same thing as "not open-source."

But a family with three daughters can be "not fully pregnant". Parts are pregnant, parts are not.

I don't think anybody would ever say a family with three daughters was "not fully pregnant." You would say some members of the family are pregnant and others aren't.

And anyway, this wasn't the sense in which Preston-Warner used the phrase — he did describe what he meant, and it was not open-source software. It was, as chimeracoder said, similar to Microsoft's "shared source" program.

A better phrase would have been "not entirely open-source".