That is not what is claimed. It is claimed that it is based on the "German mountain films" (Bergfilme) which is a totally different genre than "Triumph of the Will" or "Olympiad".
".... acutely noticed back in 2001 that James Cameron's epic movie Titanic is based on the German mountain films that Riefenstahl starred in."
The Bergfilme was as popular in the German cinema in the 1930's as the Western was popular in the American cinema in the 1950's.
I'd like to see some evidence of this acute notice in 2001.