In this case, it's argumentum ad homonym and a red herring.

How so? The initial inquiry was whether MLK was individually important or if others could have filled that role. His plagiarism is highly relevant because it demonstrates he was not originating many of the concepts and ideas behind the movement. As he didn't create many of these ideas, they were not what made him uniquely important to the movement. His assimilation and presentation of these ideas were his contribution (IMHO), and I'm not going to take a position on whether anyone else could have done so to similar effect.

An ad homonym / red herring attack would have been to bring up his infidelity, but it's not relevant. As MLK has been essentially deified in the popular mindset, there's a lot of association fallacy when discussing him. He was a great orator, but (IMHO) his writings contain too much known plagiarism to claim that he was a great writer. But I'm doubtful people will try to see through the halo effect long enough to reference his actual strengths to support their points about his significance, so whatever. He was perfect at everything. Nobody else in the civil rights movement could have taken his place because he made such great waffles.

And, so you don't have to take my word for it, here's the most commonly quoted statement on MLK's plagiarism:

Stanford History Professor Clayborne Carson, the Director of the King Papers Project wrote the following passage:

"When our research was published in June 1991 in the Journal of American History, the article made clear that King’s plagiarism was a general pattern evident in nearly all of his academic writings. Although the plagiaries in the dissertation were less egregious than the press reports had suggested, they were more extensive throughout King’s papers than had been reported. We found that instances of textual appropriation can be seen in his earliest extant writings as well as his dissertation. The pattern is also noticeable in his speeches and sermons throughout his career."