Smerdis was a magian.
For example, the book Darius the Great by Jacob Abbott (freely available online e.g. here and here) has a chapter entitled "Smerdis the Magian", and the journal article Arno Poebel, "The Duration of the Reign of Smerdis, the Magian, and the Reigns of Nebuchadnezzar III and Nebuchadnezzar IV", The American Journal of Semitic Languages and Literatures 56(2) (1939), pp. 121-145, also refers to him thusly.
What is a magian, you may ask? There's an entry on Dictionry.com (mixed up with magi), and a much more detailed description here. In short, they were important religious figures in ancient Persia, not magicians.
So why did Borges call Smerdis a magician? Well, did he? Remember this is a translated story, originally written in Spanish. Checking the Spanish text, we find the original version of this passage (emphasis mine):
De los nombres históricos, uno solo: el impostor Esmerdis el mago, invocado
más bien como una metáfora.
The Spanish word "mago" does translate as magician, but as far as I can tell from browsing Spanish-language pages about Zoroastrianism and ancient Persia, it's also the Spanish word for "magian". E.g.:
En la religión tradicional irania, los sacerdotes eran llamados magos.
-- National Geographic
Según Heródoto (I-101), los magos eran una de las seis tribus de la Media.
-- Spanish Wikipedia
TL;DR: it's a translation error.