Complex sentence: The followers of the Thule Society were, by Sebottendorff's own admission, little interested in occultist theories, instead they were interested in racism and combating Jews and Communists.
Split into simpler sentences: The followers of the Thule Society were, by Sebottendorff's own admission, little interested in his occultist theories. They were more interested in racism and combating Jews and Communists.