It looks like the Extendscript documentation for the book method (page 599) has been swapped with that for the document (page 714), so the note about ImportFmtInclude is missing for the book, but is superflous for a document. I have created a bug report for this documetation error FRMAKER-10499
The other part to this error, is how can the ImportFmtInclude be cleared or set, through the user interface? It is easily manipulated from Extendscript or c++, but I can find no reference to it in the user manual, at least for 2019.
Also, is it a bug that a generated document, the table of contents, which is unstructured in my customers environment, does not have this property cleared?