I think I may have found the answer to my own question, and it's more interesting than I expected!The Yogini's Eye
is apparently the foundational textbook for tantra in the Sakya School, and its Chapter 4 contrasts Sutra, summarized as the path of renunciation, with Tantra, the path of non-renunciation. (I've mentioned this in my most recent blog post: http://meaningness.wordpress.com/2013/1 ... -buddhism/
Rinpoche's academic training was of course in the Sakya School, and I suspect this is the explanation he drew on. Since I previously knew almost nothing about Sakya teachings, I wouldn't have thought of this as a possible source.
I found The Yogini's Eye
to be an exceptionally clear presentation of tantra. It's opened up a new dimension of understanding of Rinpoche's thought.