The barest essence of Capitalism is free commerce, conducted under rule of law, and that no individual has more rights under the law than any other.
A black man in that era, was not equal under the law here. At the same time, communist agents could point him to Moscow and tell him how free blacks were there. That they were integrated into universities. That they were equal under soviet law.
He would neglect to point out that the USSR essentially HAD no black population beyond a few students from Africa. He wouldn’t point out that equality under Soviet law was a worse life than 2nd class status here in America. He wouldn’t point out that Russian people never consider blacks their intellectual equals.
But in that era, asking a black man to celebrate capitalism when the biggest capitalist nation refused him equal legal status, was probably a bit much to ask. I say we know he was a flawed man, like most high end performers, that he was fighting for equality, and leave it at that. He was seduced twice. By women, and by socialists, but his goal was the end of 2nd class status. Both seductions hurt that goal.
And btw, Charleton Heston agrees with me that King was on the right side of the argument.
I understand that MLK was both a capitalist and a Republican.
Also, my recollection of the time is that two speech writers for King, whom traveled with him, were definite Communists. We may little attention to it today but, in that day and time, being Communist was a big deal.
And again leaning on recollection, King knew they CPUSA types, but kept them because they were good at what they did.