It’s clearly not plagiarism or churnalism – but was it an error in another way? Yes. I now see it was wrong, and I wouldn’t do it again.
Why? Because an interview is not just an essayistic representation of what a person thinks; it is a report on an encounter between the interviewer and the interviewee. If (for example) a person doesn’t speak very good English, or is simply unclear, it may be better to quote their slightly broken or garbled English than to quote their more precise written work, and let that speak for itself. It depends on whether you prefer the intellectual accuracy of describing their ideas in their most considered words, or the reportorial accuracy of describing their ideas in the words they used on that particular afternoon. Since my interviews are long intellectual profiles, not ones where I’m trying to ferret out a scoop or exclusive, I have, in the past, prioritised the former. That was, on reflection, a mistake, because it wasn’t clear to the reader.