For a time in the 9th century the city appears to had some population of speakers of Old Church Slavonic. Saint Cyril and his brother Methodius were born in Thessaloniki and the Byzantine Emperor Michael III, saying that "the inhabitants of Thessaloniki speak Slavonic quite well", encouraged them to visit northern Slavic regions from there as missionaries.
which I consider misleading. In particular, it is the first sentence mentioning speakers of Old Church Slavonic which I think is the source of the controversy. For more details, you may refer to the talk page of the article.