Isn't it difficult to do a lecture on any kind of opening without being somewhat biased towards it, if promoting it?
I haven't seen his lectures, I've only read the book. If it's a live lecture on an opening in general, it might be permissible to miss some very minor lines here and there, as viewers' attention spans are limited, though the speaker should avoid doing that. It is not, however, good to miss key lines and ideas in an opening book, which is what Dzindzi and Alburt do in their book. Additionally, a slight bias might be okay, but an overwhelming one is not fine if it completely skews the reader's perspective to a point where it's obvious that they're wrong.

Isn't it difficult to do a lecture on any kind of opening without being somewhat biased towards it, if promoting it?
It is not. A brilliant example are the recent DVD's of Svidler on the Grunfeld, and P.H. Nielsen's on the Dragon. Great, top notch GM analysis, useful even to very advanced players, and yet accessible to anyone. Not everyone can produce such stuff- and Dzindzi surely can't.