Just a bit of context. There are multiple historical biographies of Saint Naum of Ohrid. The first was issued in the 10th century and the second in the 16th century. Both of these were produced in Macedonia and written in the so-called Church Slavonic (i.e. Medieval Macedonian) of their respective periods. Then there are biographies written in Greek from later periods. The dates of some are contested and some contain inaccurate information - except the first one. The quote you mention comes from the 16th century biography. Here is the full paragraph:
A few points. The reference to Saint Naum growing up in "Mysia" suggests the terminology in the text may have been influenced by a Greek source. There is no evidence that he grew up in Bulgaria. As part of their mission, Cyril and Methodius taught in Moravia and Pannonia. The author is either using esoteric terminology or referring to the continuation of the missionary work by their students and those that followed. Further down the text, it refers to the Bulgar ruler Boris as a "tsar", even though he was never referred to as such whilst alive. It also refers to the "Bulgarian" language or script, even though nobody during that period referred to the language or script of Cyril, Methodius and their disciples by that name. None of the above is mentioned in the first biography, which was written decades (rather than centuries) after the passing of Saint Naum.
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