Are past perfect and simple past interchangeable when describing events that happen for some time in the past?

I'm using an exercise from the Purdue Owl to understand tense usage. The exercise, which uses an excerpt from Alex Haley's Roots, can be found here: https://owl.english.purdue.edu/exercises/2/22/51

Here's the sentence I'm having trouble with: "Since my forefather had said his name was Kin-tay (properly spelled Kinte), and since the Kinte clan was known in Gambia, the group of Gambians would see what they could do to help me."

In this case, Haley uses both the past perfect (had said) to indicate something that started sometime in the past and continued up to another time in the past and the simple past (was) to describe something that was true for some time in the past. It seems to me that in this case, the past perfect and simple past tense are interchangeable. Haley could just as easily written "since my forefather said his name was Kin-tay" or "the Kinte clan had been known," although the latter example wouldn't have been as graceful. Are these two tenses sometimes interchangeable? Or is it a matter of duration? Have I missed something?

Answer

In the sentence you present:

"Since my forefather had said his name was Kin-tay (properly spelled Kinte), and since the Kinte clan was known in Gambia, the group of Gambians would see what they could do to help me."

The use of "had said" indicates that the forefather's action was already in the past at the time of the events then being described by the narrator. If you wrote instead "Since my forefather said his name was Kin-tay [...]" then the natural out-of-context interpretation would be that the forefather was present and gave his name in that moment, thereby directly influencing the Gambians to help. That the reader will recognize that interpretation to be impossible doesn't make it ok to substitute "said" for "had said" here; although the reader might understand the author's intent, the conflict between context and natural interpretation would be dissonant.

Are these two tenses sometimes interchangeable? Or is it a matter of duration? Have I missed something?

No, they are not interchangeable. It is a matter of the relative timing of events. When describing (simple) past events, one uses the past perfect to place actions in a more distant past time frame.