I think your subtitle to the article is an overstatement in itself. "A U.S. briefer overstates Iran's meddling in Iraq, setting off a Washington tempest."
After reading this article it became clear that the briefer did not overstate the "meddling" (I question whether such a cute term should be used in relation to the killing of our troops) by Iran in Iraq, just that he might have overstated whether there was proof that the "meddling" was expressly authorized by the highest authorities in Iran.
This story is not really newsworthy.
It is clear that the government is of one mind that the Iranian Quds Force is involved in the meddling. It is also well known that the Quds Force represents the elite special forces of the Iranian regime (like a mix of SEALs and CIA covert ops). If the government is correct about Quds Force involvement (your article casts no doubt on their assertion), then doesn't the burden shift to Iran to explain that its elite special forces were not acting on the orders of the highest levels of the government?
Isn't there a presumption (respondeat superior) that the acts of the Quds Force were authorized at the highest levels?
Even if you take the position that there is not a presumption of authorization - how rational is it to expect that we would ever be able to get hard proof of such authorization?
Read it for yourself and see what you think. I think the authors are gleefully setting a standard of proof that could never be met and should never have to be met.