The US claims that Russia is planning to make a high quality fake video showing a Ukrainian attack of some sort. But it has no proof to offer.

I am not surprised there is no proof. For something like this, it would be hard to get any.

I agree that we can't be sure the claimed US intelligence is honest. If the US were presenting the accusation as a reason to preemptively attack Russia, the analogy with the invasion of Iraq would be valid.

However, the US does not propose to attack Russia. Quite the contrary, it doesn't even commit to help Ukraine defend itself if Russia attacks. So the purported analogy is just an insignificant coincidence.

I am not sure what purpose the US has in making this claim. If the claim is honest, perhaps the aim is that the world disbelieve the Russian phony video footage, if such footage appears. If it is not honest, I can't see any point in it.