I will be happy to engage Drexler at length when I get the chance to do so. I have not, in the last 3 days, managed to buy the book and go through the physics in detail. I hope that failure is not enough to condemn me as not acting in good faith.

Certainly not (perceived as acting in bad faith). Instead, that particular comment was a misstep in dance of rationality. It was worth correcting with emphasis only because many other people were making it too (via excessive upvoting). As Eliezer noted there would be a big improvement if you said "oops but still consider the PR implications".

Like Carl I would appreciate someone else analysing the physics in Drexler's dissertation and book thoroughly and giving a brief summary of key findings and key concepts.

I personally would not, when giving an explanation for some phenomenon, ask you to take for granted without at least a citation the following statement.

For my part what I do take for granted is that DNA based machines can be used to create arbitrarily complex impacts on the environment. The question of precisely how much smaller than DNA based cells it is possible to make machines is a largely incidental concern.