I'm not an expert on Yoga by any means, but in all the classes I've taken (and with the exception of the "relaxation" poses), the instructors have emphasized maintaining a straight back. Spinal flexion hasn't been explicitly encouraged, unless (perhaps) it was in the context of a forward fold, where the goal is to relax the muscles of the back after some other strenuous exercise.

Also, reading through your links, the McGill fellow seems to have a fairly nuanced position on core strength. It's not as simple as "don't bend the spine forward".