Hey there!  Resident Lore scholar here.  First off, a questions like this fits better on our forums rather than here.  Though I will give a quick answer.

What you heard was false information and got mixed with theories.  "Dovahkiin" and "Dragonborn" are two completely different things, and the theory was only talking about Dragonborns, not the player character of TES V. 

If this answer does not satisfy you, you can PM (Private Message) me, or start a thred on our forums where there can be discussion. 

Again, the question board isn't for any sort of debate, and this is not the proper place for this question.  Thank you!

Its probably just an ingame myth. Or it could be them trying to summon him.

However, a large chunk of the dragonborn's old life is unknown, and he could potentially be that old, just with amnesia or something. This seems unlikely, as you start from low-level skills and magic and someone who is roughly 200 years old should be pretty experienced.