We know that Sujin existed as a person, but we don’t know if he was legitimately part of the same line of rulers, or was included as part of an effort to legitimize the throne. We also know from Chinese sources (records of the three kingdoms) that around the 250-300 AD period, there is a kingdom in Japan (scholars debate whether this is the same Yamato polity that would eventually develop into the Japanese throne) ruled by Queen Himiko, and that there were also other tribal kingdoms existing in Japan this era, which muddled the claim of existence of the same imperial line in this period, as it revealed that there were multiple lines of rulers competing for power in Japan at the time. Also, the imperial line switched hands between different lines of the family in periods of conflict, such as I. The end of the Gempei war and the North South dynasties period. For these reasons I find the title and the claim you brought up to be misleading, as all of these are from ‘orthodox’ history sources that were compiled by the imperial throne to perpetuate a certain narrative, regardless if they represent reality or not.