Regardless of where he said it, "dumbwhite*" is a phrase with racist intent. Objectively based, "fucking black people" is just a reference to people with a dark skin color but in the context of the English language we all know what it means.
I agree it's not his job to educate, and he shouldn't have to. But in reality, most racists are not going to educate themselves. If they did, they probably wouldn't be racists. So if he actually wants to see a positive change and stop Steve from making racist comments, his best bet is to try and educate Steve himself.
This article might just do that. The article itself already talks about how Steve was completely dismissing everything he had to say. But in written form with a semi-anonymous target, the article will likely filter back to the company and to Steve. In written form, the author gets to say everything he wants to say and why. If Steve reads it, in all likelihood he will dismiss it, but it may also provide a nugget around which he may start to see what he was doing.