The matter of who created Felix the Cat is controversial, based on what I've read from the most serious researchers, I believe the character hasn't been created by Sullivan. But it's your view and you're entitled to it
You're right, it's more than likely that it was Otto Messmer who created Felix, however, there's no doubt that Pat Sullivan was behind Felix's popularity, as he had a flair for hiring the right entertainers like Bill Nolan, Burt Gillett, George Stallings and Raoul Barré to name but a few.
Although I quite agree with what you say, I still find you very pessimistic about the possibility of a black animator at that time.
The career of Chinese-born animator Cy Young proved that there was slim hope for minorities who wanted to do animation, Young even managing the animation department at Audio Production. Paul Terry was also very progressive on this issue, so much so that one of his animators speculated that he would have hired blacks in his studio if he'd had the chance, which is quite possible to envisage on Terry's part, especially in the early '30s when the need for manpower was soaring.
Cy Young had been lucky enough to be able to experiment with the art of animation before he left for America, which is why he was easily able to get a job as an animator in the 1920s - quite the opposite of this mysterious African-American who probably wasn't even an artist. Walt Disney would never have hired Young if he'd known he was Asian, and when Disney realized this, he decided to isolate him in the small special effects department to separate him from the other white animators. Because Disney had a very conservative mentality, believing that minorities should serve the white man, the idea that an Asian animator could achieve a higher status than his white colleagues was something that frightened him, since in his mind, he was convinced that only white men could make cartoons, and this was also the position of many company directors at the time. Cy Young was eventually fired from Disney Studios and never found a job in animation again, ending his life miserably, even to the point of suicide, so it's not a success story...