Satan did exist in the old timeline, but it was just 'symbolic' though. We had "Satan's Ring" in AoS. Satan is all about the "dirty work". You do something bad, it was in the goodwill of Satan because that's what Satan wants, like Dracula... As long as there is evil, Darkness like these corrupted souls will triumph. In the OT, one can consider that Dracula gets his powers from darkness which is Satan's domain and he uses his subjects to do evil for him. Wars, Rapes are done by humans though "Evil Thoughts" which is projected by Satan in most Abrahamic religions and that favors and strengthens Satan. Satan in my opinion in Castlevania probably favors Dracula, like Hades to Ares. Though Satan and Dracula probably never met, as long as you have beings like Dracula causing evil in the world, Satan is active and he can just lie back and relax.
In LoS, Satan probably decided to take matters in his own hands, and used his puppet for a while and then he reveals himself and this is what LoS is. It's the ultimate battle between good and evil. Now that Dracul is born, he probably fears that Satan may have control over his spirit like he did over Zobek because of his Dark Powers. But that's just a speculation.