It sounds like Daniel Tammet is anything but an idiot, but he is definitely an autistic savant. His insight into his abilities are described in this fascinating article in the Guardian.
Daniel’s facility with languages and figures reminds me of a man called Shiva who I met in Goa last year. Shiva is not not in any way mentally disabled, but he is fluent in 15 languages (last time I asked - probably more now). That sounds like savant capabilities to me. He has the most healthy mind I have ever had the privilege to encounter. With my tight subject focus, I almost feel autistic in comparison. [sigh]