That European Spanish phonology includes the /θ/ sound for z, ce, and ci does not make it a “lisp.” Spaniards can and do have the “regular” /s/ sound just like other dialects of Spanish. It would be just as silly to say that English speakers who say “think” instead of “sink” speak with a lisp when they are two different words separated by a minimal pair

A single person with a lisp (even a king) cannot influence a language spoken by millions of people. Languages change via natural processes over time, the king thing is an urban legend