In fact, doesn't a denier automatically, by his mere existence, claim that "the others" are "zealots"?
True enough. However coming to a discussion with no opinion is next ot impossible for someone to do. It's reasonable to imagine someone who follows the evidence will have a pre-conception of what the answer is.
What sets him/her apart is a willingness to be proven wrong (or even just a bit less right).
If you already know the answer to some complex issue that is not direct, hard science, then you are cognitively limiting yourself. If it's reproducible and demonstrable, of course, then we can still remain properly agnostic and rely on induction to save the day. If it is not, we have to be very, very careful about what our hidden assumptions are.
Also it's interesting that in that particular debate, there are "deniers" but no "zealots". It's only one side that feels the need to label the others. That ought to set the alarm bells ringing.