Wow, that is definitely non-standard sentence diagramming. It looks a bit hinky to me. If nothing else, shouldn’t that “or” be hoisted a bit with “accurate” and “not” hanging off of it. I’m guessing they are doing a strictly syntactical parse, not a structural parse.
They stopped teaching sentence diagramming in the late 1960s, so I’m not surprised by the test sentence. It’s a real pity. The kids have to guess now, and hope that their parents had good enough grammar so they can rely on their intuition.
