First of all, thank you for reading this question. While checking some proposed restorations of Indo-European words, I noticed that for Slavic words the Old Church Slavonic is used. I've searched some information of this language and found out that it was artificially created on the basis of Thessaly dialect of local Slavs.
So the language was invented by two Greek monks on the basis of local population, that consisted not only of Slavs but slavicised Turkic Bulgars, Greeks and native Thracians, that according to some sources had a lot in common with Balts in terms of linguistics (the same is usually found between Baltic and Slavic languages).
The Old Church Slavonic was the first literary language of Slavs, that was purposed for religious texts translations and other liturgical purposes, so the language influenced all the Slavic languages. So could all the words with Indo-European roots just be calque and loanwords from Greek or Thraci? Of course the language consists not only of vocabulary, but also grammatics and other components, but I don't have much knowledge about it to compare myself.