Here's what I found, in a chapter titled "Is the Old Testament Still a Hellenistic Book?" by NPL:
Here we have it. All that argument from Bickerman's article that I dwelt upon -- and now NPL simply tells us in a half dozen words what we sometimes find in the literature: primary sources are sometimes found embedded within a secondary source.The problem for biblical historians was, and still is, that they have tended to identify the oldest source found in a written document with a primary source, understood as a contemporary source. To a great extent, they have ignored the fact that also the oldest part of an ancient document may be a secondary source! They have been absolutely blind to the claim of the “father” of Danish historians, Christian Erslev (1852-1930), that secondary sources are also of value if they are read as testimonies from the time when they came into being. This also implies that primary sources embedded in a document, which can only be considered a secondary source, are likewise part of the outlook of the people who drafted older information into their own retelling of the past.
But NPL did not have to contend with critics on BC&H, so I still think it worth being careful with words and noting that it's the "intellectual content" of a primary source that is embedded in a secondary source.