that is the fairy tail by kairi's grandmother, while the keyblade war legend supports it we dont know exactly how it happened also we have the pureblood heartless to support that whereas we dont have any nobodies.
We can pretty much take that origin story as basically true given all of Xehanort's research and writings on the subject. Kairi's grandmother's story was a bit more of a fairy tale version of it, but it was always rooted in the actual history of things (or else why bother including it in the game?), and we don't have any other history to go by at this point. It doesn't seem to be a point of contention within the game itself (none of the characters have stated it wasn't true, and again, Xehanort's research supports it), so why question it?
not that there isnt any but if there has always been nobodies why arent we getting any leaking out of the RoD like the heartless?
Because Nobodies aren't born in the RoD. When a Nobody is created, they're born in an in-between world. And, as you've said, it wasn't until after BBS that the walls between Realms came undone. So, when Heartless started pouring into the RoL from the RoD, the Nobodies began to enter the RoL from the Realm Between.
also you have to ask yourself how did heartless get created up til after bbs? cause so far we know they couldnt leave the RoD (no the KG isnt of the RoL) so how exactly was they being born up til now?
A) Because darkness still exists in the Realm of Light prior to and during BBS. People's hearts were still falling to darkness, they would become Heartless, they would just be locked away in the Realm of Darkness without being able to return to the Realm of Light. Remember, ever since the World separated, there has been darkness in every heart; anyone is liable to fall into darkness if their light is not strong enough to balance it out, and they can become a Heartless. There doesn't have to be another Heartless involved in that process.
B) Heartless, as far as we know, do not age and do not die. Therefore, any of the Heartless that were born during the destruction of the one world would have still existed in the Realm of Darkness. No one was destroying them so they would have just existed, feeding on the dark nature of the Realm itself, and maybe each other.