This guy claims to be from our culture ~20 years in the future. Your question seems to be something like "Yeah, but what if everything was completely different?"
Not terribly relevant in this case since I doubt anybody is even remotely fooled, but more interesting in the Jon Titor case, where I think many people were (albeit with a certain amount of willingness). He kept his predictions close to his chest and tried to play off any inaccuracies as issues with "alternate realities", but if that is true and you're not even in your own past anyhow there really wasn't very much reason not to be far more free with the details of the timeline that we may not even be in.
I wouldn't talk to anyone: they'd die of several diseases from my breath pretty quickly if I understand the epidemiology of it. I wouldn't want to kill people, or get labelled a plague carrier!
I doubt i can ever convince someone from over 600 years ago about anything.
Someone who is reasonable well-informed, but living in the year 1371 is still a whole different level of information than someone who is reasonable well-informed today. Lots of concepts have not been discovered/invented and another whole bunch are lost.