Notice the contrast is made clear in English by using an emphatic construction:  it was not the Balkans but...    In Greek the translator has chosen not to follow this construction of the English.  I think (subject to correction) that the Greek reader has no problem in perceiving the meaning of the Greek text; however, an English reader would probably fail to understand the intention if the writer had written The Ottomans ruled not the 'Balkans' but 'Rumeli', as this sounds more like a correction of somebody who thinks they ruled the Balkans (and that the Balkans and Roumeli were two different places).