[freddyc]

No, I don't think that is correct. The US is silent on dual citizenship but I believe that it's only if you pledge your allegiance to another country (e.g. to defend that country in a war) that you are deemed by the US to have revoked your citizenship. I'm guessing that didn't happen in Thiel's case.

EDIT: to address your edit - you need to look at the wording of the relevant INA section which includes "with the intention of relinquishing United States nationality". That's the key part, you have to have the intent to abandon your US citizenship. I know plenty of US-born people who hold dual citizenship so it is definitely possible.