Meh. This is a good move by the Virginia GOP because at the very least it will establish that these gotcha moves, usually directed at the Republicans, shouldn’t be taken seriously.
As for witnessing a non-signature, you miss the point, which is to verify that yes, it is McAwful and not someone else whose signature appears. If he then failed to sign it for some reason, that’s on him, not them. It would only be a crime if, for instance, McAwful refused to sign it saying that doing so would be attesting to facts that he doesn’t know to be true, and so someone counterfeit his name on the document, and they signed it, too.
In such a case, McAwful would be the wronged party: “No, you knew that there were flaws, and instead of fixing them, you stuck my signature on it, and now I’m accused of perjury.”
Of course, because McAwful has failed to assert the facts in the document, the state doesn’t know them to be true.