Complex sentence: He pointed out that, contrary to the film, Thälmann was not on the Western Front when the German Revolution broke out on 5 November 1918, but in Hamburg: this detail was cited in Thälmann's official biography, written by Bredel himself.
Split into simpler sentences: He pointed out that, contrary to the film, Thälmann was not on the Western Front when the German Revolution broke out on 5 November 1918, but in Hamburg. That was cited in Thälmann's official biography, written by Bredel himself.