Let's see the definition for calambre as it was a hundred years ago. This is written in the Royal Spanish Academy's dictionary from 1914:
Calambre. (Del ant. nórdico klampi, laña, corchete; en al. krampf, calambre.) m. Contracción espamódica, involuntaria, dolorosa y poco durable de ciertos músculos, particularmente de los de la pantorrilla.
No trace of the electrical shock-related meaning, neither in this dictionary nor in other dictionaries from the same time. That meaning was added to the dictionary in 1992. In fact, a hundred years ago the term to refer to an electrical shoch was descarga eléctrica:
Tona sufrió una sacudida de arriba abajo, como si la hubieran aplicado una descarga eléctrica [...].
José María de Pereda, "Peñas arriba", 1895 (Spain).
In the aforementioned dictionary you can read:
Descarga. Acción y efecto de descargar.
Descargar. 5 Anular la tensión eléctrica de un cuerpo haciendo saltar la chispa o por otro medio.
So it seems that the person died indeed from a cramp. And yes, the word calambre dates from very old, it can be found in the Spanish-Latin dictionary by Antonio de Nebrija, written in 1495.
Reading this text from circa 1535 it seems that people could indeed die from a cramp:
"Pereçosa,
vellaca, puerca, golosa,
mala hembra, desoluta;
di, ¿no acabas, çancajosa?
¡Ven aquí, borracha puta!
Dormillona,
¿de dónde vienes, soplona?
¡Mueras de mala calambre!"
Anónimo, "Auto de Clarindo", c1535 (Spain).
The last sentence means "may you die from a bad cramp". By the way, it seems that Spanish calambre and English cramp have the same origin.