Complex sentence: Fennell claimed that most of the Teutonic Knights were by that time engaged elsewhere in the Baltic, and that the apparently low number of knights' casualties according to their own sources is indicative of the small magnitude of the encounter.
Split into simpler sentences: Fennell claimed that most of the Teutonic Knights were by that time engaged elsewhere in the Baltic. He also states that the apparently low casualties suffered by the knights according to their own sources is indicative of the small magnitude of the encounter.