@aaaaa There’s few other sources that mention some kind of writing system of Slavic groups. Ibn Faldan describes burial customs of the Rus in Volga Bulgaria and says they made inscription on the tomb. Of course we can’t be sure if those people he writes about were Slavic at all. Thietmar of Merseburg writes in his chronicle that Slavic idols on Rujan island had their names inscribed onto them. Panonic Hagiography of st Cyril and st Methodious also mentiones that Slavs in Crimea had their own script before Cyril and Methodious.
These are all about Slavs on the borders who were in contact with other tribes and cultures, so can’t know if that’s something that was common for Slavs or just something those particular groups adapted from their neighbors. I do agree with you that some developed Slavic literature prior to christianization is highly unlikely.