But are you going to trust aljazeera's signature on a [potential] deepfake of an American politician as the only source of truth about whether that video is real?
That isn't what signing does. Signing attests to origin not authenticity.
In a technical sense, signing only attests that a specific key pair was used to process the data. How that key pair is associated to its handler is a social function. If the signer claims authenticity, the general population will accept the claim, regardless of whether the claim has technical merit.
I'd trust more than a deepfake TicTok video reposted by FoxNews/CNN.