Looks like you're right:

"1. You are welcome to post pre-submission versions or the original submitted version of the manuscript on a personal blog, a collaborative wiki or a preprint server at any time (but not subsequent pre-accept versions that evolve due to the editorial process)."

http://www.nature.com/authors/policies/confidentiality.html

... But I can see how authors, anticipating a possible Nature publication, would choose to maximize "impact" by not jumping the gun.