The line is: "Any nondragon (cannot already be a half-dragon)."
No grammatical reading of this line makes it mean what you say it does.
In particular, trying to make the parentheticals be an ie statement results in a logical absurdity.
To dismiss an incoherence and then point out what this rule is actually doing (precluding the double application of half-dragon template benefits) is not the same thing as moving from an RAW to an RAI argument.