I'll believe that they invented it for themselves, but as far as I can tell, Bird's rhythm was his own, and more or less intuitive to him, roots in Lester Young aside. You can hear the difference in Diz' phrasing pre- & post- Bird, and the difference is significant. In fact, Diz has said more than once that he brought the harmony & Bird broght the phrasing. No doubt that's a bit of an over simplification, but it's true enough.
Bird was truly one of the freest players ever. The closer you listen to his internal rhythm (where the phrases begin & end, and how the accents bob & weave within each phrase), the more mind-boggling it gets, especially on the live stuff. And you don't have to be a musician to hear it. Not at all. You just have to have a good feel for rhythm, and you just have to be conscious of it, that's all.