As for who might or might not have been confused, I don't know how many gullible people may have been influenced. The issue is whether or not the article looked like a peer-reviewed paper published in PNAS. The NAS was concerned enough that this confusion could arise that they issued a statement of clarification.
Be honest. The letter was mailed out to 19,000 people. 19,000 people with a technical background. People who went to college and had to understand math, maybe physics, maybe chemistry, maybe biology. Intelligent people. What you are implying is a kind of mass hysteria - that thousands and thousands of technically minded people simply looked at the name "Frederick Seitz" and how the article appeared, recognized that format as a PNAS format (not just a technical paper format), and without reading its contents, signed the petition simply out of respect for the National Academy of Sciences.
The issue is that human induced global warming is not accepted as scientific fact. The Petition Project simply documents that there is no concensus on human induced global warming.