Okay, I don't know about anyone's theories because I am not an official lore maker, what I do know is grammar. Ra-den wouldn't be a Titan if he was called a Titan Keeper, he might be a Titan if he was a Titanic Keeper. As Titanic is an adjective, making him a Keeper who is titanic. Where as Titan Keeper means he is a keeper of titans. Just like a Dragon Keeper would be a keeper of dragons not a dragon who kept things, that would be a Draconic or Draconian Keeper. You can argue all you want, ignoring grammar facts or whatever, but the way its typed, or written, or phrased shows that Ra-den is not a full fledged titan, but some badass mini-titan thing that keeps titan tech or artifacts.