How much clearer can it be that he is admitting he can’t prove Sr. was his father.
He covers himself every step of the way in that book, he even has Sarah say ‘NONE OF YOU ARE HIS CHILDREN’ to Kezia’s brood. Why? Because one day, if it all comes out, then he can say ‘that’s what they told me’ - everything important is spoken by someone else. It’s worth reading just to understand that aspect.