Yeah, this is the exact myth I was talking about. The European's diseases killed 90% of the Aztec population; the actual soldiers could not cleave through native warriors by the thousands. The technology difference gave them an edge, but it wasn't that great.
I was talking to someone earlier this year who was an expert on Mesoamerican societies (specializing on the Tarascans) who was pretty damn critical of Jared Diamond's Guns, Germs, and Steel. Said the only part Diamond really got right was the Germs part.
Tarascans you say? I'd love to Chat with this person about all they know.