Some say that the early Caliphates were very important to the preservation of records of pagan mythologies e.g. greek gods by Islamic scholars, which otherwise might have been wiped out by Christianity. To what extent is this true?

Records of pagan mythologies

That's not something I've come across. The Islamic "Translation Movement" under the Abbasid Caliphs was an important avenue for the preservation and transmission of various ancient greek texts (i.e. produced by pagans, but not religiously "pagan" per se), without which, we now know, would otherwise have been lost. Greek philosophical texts in particular seem to have been well preserved by the translation movement.

As far as I'm aware this wasn't because these documents were being "wiped out by Christianity." On the contrary, Syriac-speaking Christians played a critical role in the Translation Movement. It's just that, with the benefit of hindsight, we now know that many of these texts would otherwise have been lost.

As far as I'm aware a number of what we now regard as essential Greek pagan texts were not translated into Arabic to an extent that's survived at least until the modern era, including Homer.