It was not: in theory, it should have been a discussion that involved "only" the reviewers and the AC. However, none of the reviewers (besides me) participated---thereby resulting in a 1:1 discussion with the AC (whose remarks could not have been addressed in any "revision" of the paper, since this occurred in December).

Interestingly, there is even a "gap" between the Reviewer and AC guidelines:

• according to the AC guidelines, the authors could "participate in the discussion" occurring between November 18th and December 12th...
• ...but this was not stated in the reviewer guidelines, which stated that, after November 18th, the only role of the reviewer was to have the "virtual discussion with the AC for borderline papers".

I was unaware that the authors could still "participate" after November 18th, so I did not even think that it was possible to involve them in the discussion (had I known that the other reviewers would have done nothing, I would have certainly involved the authors)