It's kind of funny that people keep saying they want Paradox games to go from Rome to CK. It'll never happen for the same reason CK2 will never be pushed back any earlier than it is: Paradox will not mimic the rise of Islam. It'd get them a lot of flak, and alienate them, from Muslim players and Islam-majority countries.
To be honest, I'm pretty sure they even said as much years ago.
I can safely say I've never heard of this before. They've always seemed fine with Islam rising before, given that they even added playable Muslims early to CKII and given how powerful the Ottomans are in EU4. Do you happen to recall where they said this years ago? The only quirk in CK2 I've heard of that is tailored to (primarily Sunni) Islamic sensibilities was Muhammad's portrait.
CK2 won't be pushed back any further for a much simpler reason than Islam: it's already barely able to deal with the era it covers. Its core systems are very limited in both geography and chronology, and it shows. Even in Christian Europe, tanistry was a pain to add and the rota system is right out. Old Gods was tricky enough, but the timescale expansion in Charlemagne already pushes the limits of the core mechanics. Handling the fall of Rome in CK2 is best left to mods. For Rome, I'd be surprised to find it any better. The decline and fall of Rome and especially the Great Migrations are things that would probably go better in their own game.