Have also not read the book but my understanding of it is it definitely puts more emphasis on blackthorn and his perspective. Personally I don't totally buy the idea that Toranaga only keeps him around out of amusement and I think if it was that simple he'd have actually let him gut himself and had a chuckle about it on the way to dinner. That said I do think that monologue was to reemphasize the point that while he may have bumbled into being useful once or twice: Anjin is ultimately a mostly passive player in the history playing out around him and despite all his suffering and ambitions he is destined to be little more than a footnote.