Well, Samuil was half Armenian IIRC, but he was a Bulgarian Czar nonetheless. The people who created the Cyrillic alphabet were also working in the bulgarian schools of Ohrid and Preslav. It was probably compiled in Preslav, because down in Orhid they continued using Glagolithic for a while longer, but both were Bulgarian schools who codified the literary Bulgarian language of the time – there’s no doubt about that. Only later, as slavic literature spread both west and north did Church Slavonic appear in its localized forms, but it remained fundamentally a south slavic language – i.e old Bulgarian.

P.S: The Mongols were/are awesome. Tatars is what the Russians named the Mongols what settled around them.

Cheers, JJ