Not at all. Major passages of "Roots" were plagerized from "The African" written in 1967. Haley's lawsuit settled for $650,000, and he admitted it.
Plagerizing doesn’t indicate whether the original source material is fiction or not. It just says the written account was copied from another source without credit. So I don’t exactly see your point. The material Haley stole may be the only part of Roots that is in any way accurate.