No, we don’t know the accuracy of a passed-around email.
But Leno was retired from his prime position while still young by such standards and at his peak, which was bizarre for a network to do. Beyond understandable that he ended up leaving the gig altogether.
And he was at least more politically balanced, with some shots at Obama, than any of the leftist toadies put on in the evening by the networks since.
I saw an interview with Leno about it and Leno was respectful and gracious.
Also, I don't remember Leno making a joke about Obama although there was plenty of material. I think that he knew that it was risky and that he would be labeled a racist.