Joseph, in my humble opinion, when someone plagiarizes another person's work, they know they are plagiarizing. The focus of the article posted by the Forum was the involuntary plagiarism of another's work. The reasons cited included failure to state your own thoughts [in your own words], and failure to properly attribute your sources.

This is not the same as the case you mentioned about Alex Haley, who admitted that the plot, main character, and scores of entire passages of text [practically the entire content of the novel Roots] were lifted from the novel The African, a 1967 novel by a white author, Hal Courlander, in addition to lifting other ideas, thoughts, and words from the novels of others.

In your scenario, the words would probably be judged according to the context in which they were written. That is, while you may come up with the same thought [which cannot be plagiarized] another writer has, the words you use to express that thought, and the context within which it is expressed, can help to determine if you did, indeed, plagiarize another's work. Again, just my humble opinion.

Spivey

P.S. Hmmmm...... he could have given credit to the original author, such as a screen credit that said: based on a novel originally written by......