Earlier quoted context omitted.
It also depends on the court in question. Ie in my recent criminal Jury experience; if i were to pretend deepfakes were used then it wouldn't be on the prosecution to prove that the video is without a doubt real, but rather that a "reasonable person" would believe that the video is real. Which is quite open to interpretation. For a Jury i think it would come down to how convincingly the defense argued that the video…
I think the reasonable person standard is used for evaluating whether someone's behavior is okay. Like, if you tased someone because they were suspicious, the question would be whether a reasonable person would conclude they were suspicious based on what they did. I think you're thinking of reasonable doubt?
For additional context, the criminal case i was part of the Jury for was handed a book of definitions for the given case. Things like Assault had qualifiers of Self Defense and Stand Your Ground. Each of these little nuance-y definitions included very implicit definitions of say, the force you use in response to assault for self defense or stand your ground. This responsive force was instructed to be judged against something like "Force that a reasonable person would not believe to be in excess based on the attack".
Ie "reasonable" was only able to be decided by a hypothetical "reasonable person". Whether something is reasonable or not was only applicable in the context of the person who is deciding if it's reasonable, also themselves being a reasonable person.
Just about every definition page had an explanation quite similar to this. I think due to the fact that human definitions are very difficult to define. Did you hit them too hard? Well we don't have pounds per square inch of your delivery, or w/e concrete definition could use, so the legal definitions were bounded by this hypothetical "reasonable person".
It was interesting.
edit: I should add, in case it's not clear, that we the Jury were instructed by the judge on the legal definitions. That was the book. We were to view the case through the lens of these legal definitions with the evidence presented. What specifically we were deciding was very important and meticulously managed by the Judge.