The name Clemens Forell is an alias. The real life version of Forell was named Cornelius Rost. He used a different name as he was afraid of potential trouble with the KGB when the book was released.
Although the movie is presented as a true story, it is believed that Rost aka Forell (who truly made an impressive escape, even if not the one presented in the movie) fabricated or exaggerated some details, and claimed other people's experiences as his own, to make a good story better.