Only if you hold to a rigid 20th century view of "exclusive original
autograph authenticity". Maybe it WAS added to the LXX, but who is to
say that is not divine intent? After all, Jesus quoted it, as you
pointed out. And isn't "exclusive original autograph authenticity"
somewhat begging the question, or at least a manufactured presupposition
when we can't even produce the original, and the Masoretic evidence
weights toward LXX addition?
Don't know if we can really come to an answer on this one, but I had
these questions for further thought and further clarification.
-- Brian W. Neuschwander