I think Safi'jiiva is not exactly a Forbidden Dragon yet like both Fatalis and Alatreon does, but definitely a being that got close to them in power. Remember MHWorld: Iceborne intro cutscene of Alatreon shows the blazing black dragon to be able to froze the shed skin of Safi'jiiva completely and shatter those frozen shed skin as Alatreon fly nearby it. Despite said shed skin is shown to be able to withstand Safi'jiiva's King's Drop, as we're suggested to hide behind rocks covered with it to protect us from the large scale attack. Which means by default setting, Alatreon simply have stronger firepower than that of Safi'jiiva.