The irony is that the process is so convoluted and subject to so many external pressures, it is just as likely there were African American actors who gave better auditions and were offered the role, but turned it down for other better paying or more prestigious options. There was no guarantee this film would be a hit or even get made at the time they were casting it.
But Peele is not gonna say "he was really our third choice and we cast him because he was available." The pretense is that they "chose" to cast him, but the reality is a lot more complicated, so does it really mean anything? The real news is that it was a big hit and even that doesn't mean we'll see more movies like it.
This is actually more interesting, though, again, is it perception or reality?
Black British actors have spoken repeatedly about feeling the need to move to the US to get meaningful parts, thanks to the lack of diverse roles in UK film and television. In October, Oyelowo told an audience in London that black people’s experience had been “expunged” from the version of Britain shown onscreen, and he felt he had to leave to get work. “Please stop this talent drain. You have to change the demographics of the people who are making these [casting and commissioning] decisions,” he said.
The London actor Idris Elba, who also made his name playing American characters on US TV, has addressed the British parliament calling for greater diversity in the UK media. Last month, shortly after being awarded his best supporting actor Oscar for Moonlight, Mahershala Ali joked: “I’m just so fortunate that Idris and David Oyelowo left me a job. It was very, very kind of them.”
My perception of BBC shows and British films is they have more diversity in the casting, but I also have to say, except for Lock, Stock and Two Smoking Barrells, I really have no idea of what the Black British experience is. The closest I've seen to a movie about that is BELLE. I haven't seen A UNITED KINGDOM, though. It almost seemed like a mirror image of LOVING (also with two American characters played by non-Americans).