User blog comment:Lia Schiffer/408: Deicide 10/@comment-44061-20100617045928

Whats with the edging toward crack theories. Isshin never said he had more knowledge then anyone. Most any authority in SS would know about the Dangai and to date no one knows more then Urahara. Also who wouldn't know about Ichigo's achievements, all it takes is eyes. Ichigo has fought out in the open all over Karakura town and everywhere else its not hard for a spiritually aware person who wanted to keep tabs on him to do so.

I dont think he mentioned the former use of the dangai as any possibility beyond explaining its use and characteristics.

Until there is something that says that stating baseless things is a crack theory. Besides mentioning Zangetsu was Ichigo's there is nothing to prove in the series that any such thing is possible. We dont know why he knows what he knows or why he can do a getsuga tensho. Other then that the battle was determined between him and his zanpakuto spirit which is zangetsu not hollow ichigo.