Maybe, but then again, why write an essay condemning Communism as antithetical to Christianity if he was one, and explicitly state they will never reconcile their differences, if he was a Communist? Even under Taqqiya-type lying, that has absolutely no tactical or strategic value for the Communists, especially when making very clear that fundamental truth that Communists and Christianity simply cannot mix together would destroy any potential propaganda value of converting Christians into Communists. In fact, if I were Harry Belafonte, a Communist ideologue in other words, and I saw MLK wrote that, I’d scream at him as being a traitor to the revolution and shoot him dead for publishing that paper.
Besides, going by the argument “if it walks like a duck...”, you could also argue that Abraham Lincoln is a closet Marxist for doing correspondence with Karl Marx, not to mention Horace Greeley being one of his key members of the Republican Party (and Greeley is obviously a Karl Marx fanboy based on his hiring him and Engels for correspondence and advocating for many of the same radical reforms). Hope you’re prepared for that possibility.
And let’s not forget, even Jim Jones, whom you cited was someone who posed as a Christian in order to manipulate his flock into becoming Communists, nonetheless pushed for Socialism in his sermons, even falsely implying that adhering to Socialism makes one sinless, something that not even MLK dared try to do and if anything went out of his way to state Communism is incompatible with Christianity and must never be embraced (and that’s even more strengthening since both were operating around the same time). If anything, Jim Jones did far more taqiyya stuff than MLK ever did.
You can choose to see what is there or choose not to see.