I'd call the KC-10 a derivative of the DC-10. None of the KC-10 airframes were previously airlines. They were built and sold directly to the USAF as airiel refulers. The article makes it sound like these fire tankers are old airliners.
Modifications were done in production, apparently not to already completed aircraft, but it's still about 90% a DC10 freighter. (There have been suggestions about pulling some retired DC's out of mothballs and doing it over again)
"Derivitave" certainly but we're parsing.