The notion that we visit Dagon's plane (The Deadlands) Casts some doubt on this Info. I hope maybe this is just an example of cross-referenced information getting muddied by people who don't know the lore.
Essentially, Soul Cairn is a plain of Oblivion that belongs to no Daedric Prince. So I could see someone believing it's Dagon's Realm, Molag Bal's Realm, etc.
Everything else I hope is true, especially the new Enemies and the fact that Dawnguard is designed to eat up low-level players.
Let me quote it:
"Your quests for Dawnguard and the vampires will take you to several locations, and one of them is(light shines on the word, making it obscured), the plane player visited multiple times in Elder Scrolls IV. Unlike the fiery fields(light again obscured the information from the scan).
We are returning to Mehrune's Realm. Fact.