Looked it up, and apparently the lovely story is just an urban legend.
http://www.spanishdict.com/answers/192063/why-do-you-speak-spanish-with-a-lisp
I do know our Hispanic employees in southern CO were immensely amused by our office manager who had been a translator at NATO and spoke perfect Castilian, complete with the th pronunciation of certain words.
She, OTOH, thought they sounded like utterly uneducated hicks.
Pretentious snobs exist in every culture, I guess.