And you can build a camera that lies about this.

True, but today you also can hire a lookalike to some target person and record footage of them doing something terrible, or edit footage carefully enough that the edit cannot be detected.

Deepfake makes it less expensive to create fake video of someone, special signing cameras could make it expensive again in the face of that.

Not quite as you can't necessarily clone the keys from an existing camera without some difficulty, however you can say, replace those sensors with your own inputs fed using a cheap microcontroller you wrote the firmware for. There's a time and effort investment but it's not exactly high...