The game tells me I become a vampire, not a sion, so I haven't experienced what you're asking.
Zos messed up
I don't believe they messed up, I merely think you're misinterpreting it.
From how I read it, no less from how the cermony itself plays out, you're by all definition a vampire. Scion is merely a title bestowed upon the vampire by Lamae Bal. They hold no other function, much like how any other title of 'nobility' does.
Its quite clear that the book is written in such a fashion that they slander vampires that is not of their own blood, quite similar to both the Berne Clan, the Cyrodiilic Vampyrum and the Volkihar, all of whom, much similar to the Blood Matron's Cult, consider themselves superior to other strains of vampirism.
Its legit lore epeen stroking, quite literally. But there is not much more to it.