That is correct. Amazon will modify all books sent to their PDOC service, to mark them as PDOCs.
Sending via calibre is technically tricking the Kindle, since calibre imitates the Palm headers of purchased books.
...
Bu that isn't the question you asked. You asked if there was a way to share PDOCs via Family Library.
So it is understandable
that we were all confused as to why the suggested solution wasn't good enough.