I think OP is on to something here. In particular the study design makes it hard to tease out issues specific to common knowledge verses anchoring or bias towards ones own knowledge. Notably each individual's information set makes B look worse than A or C.

I think the study would have been more compelling if the common knowledge favored A and C but each individual's total knowledge was neutral between the A,B and C. If results favored not B in that set up it would indicate that folks were specifically anchoring on the common knowledge itself rather than anchoring on their initial hypothesis.