Your original claim was that we can't decide what "information" is, and therefore "misinformation" can't exist. To be consistent to with claim, I think we would need to hold that we simply can't know whether the deepfake is real or not.You can do whatever you want. The politician can come out and say it is a deepfake. If sone people believe it is real, it is what it is. I’ll tell you one thing, the government saying it’s fake won’t make them believe it’s fake.
(And to be clear believing in the existence of misinformation doesn't commit us to the claim that the government should regulate what individuals are allowed to post. The truth or falsity of that claim can't be used to determine whether misinformation exists or not )