Old Church Slavonic is the oldest attested Slavic language. In linguistics that means it’s the oldest variant we have written remain, showing us a form of the language. Thus a cursory search will always result in showing it as the ‘oldest’ form of a language. But what I’m more interested in is where did you read (New) Church Slavonic is the oldest Slavic language?

We know Cyril and Methodius had started standardising the language based on the Moravian dialect and the Slavic tongue they knew from their home city fo Thessalonike by 852 AD (Curta 2006). And these are simply the oldest attested inscriptions thus far. As you mentioned above, Višeslav was a Slavic speaker as were his people in the 8th century AD.

In fact we know that the Slavic languages had become distinct a distinct dialect from their closely related Baltic cousins by 1500 BCE and by 500 BCE a ‘Pre-Slavic’ phase was entered, where influences from East Germanic and East Iranian languages (via Gothic and Scytho-Sarmatian influences respectively) and they formed a separate language by 300 CE when the Sclaveni and Antes become regularly attested as separate from the Germanic tribes in Roman sources. So the hypothetical, reconstructed mother language of all Slavic languages is probably dated to 400 AD (Koblyinski 2005), the last time they would have been mutually intelligible dialects is around 600 CE and the first written inscriptions are of Old Church Slavonic in the Galloglitic script, c.850 CE.

Sources

Curta, Florin (2006). Southeastern Europe in the Middle Ages, 500–1250. Cambridge: Cambridge University Press.

Kobylinski, Zbigniew (2005), "The Slavs", in Fouracre, Paul (ed.), The New Cambridge Medieval History, Vol. 1: c. 500–c. 700, Cambridge University Press