The terms of service is unclear about whether I own my own submissions.
I read this the other way around. Nowhere in the ToS are you (as a content provider) assigning Copyright in your material to Steemit. Therefore if it's your original material, you own the copyright (in accordance to the law of your jurisdiction).
Is this considered "Steem Content"?
I read Clause 3 of ToS as Steemit saying, "there is a whole bunch of material on this site (Steemit Content) that you can read it or consume it for personal but not commerical use." It looks like an attempt by Steemit to cover their asses, from authors/ content providers claiming that Steemit help enable a breach of copyright.
I agree it could also be clearer that you retain copyright to your work.
Ultimately however I rely on commercial realities trumping any legalities or ToS. It is not in Steemit’s commercial interest to attempt to deny anyone the ability to re-use their own work. Steemit would go out of business real fast as people would stop using the platform, if they went down that route. Besides Steemit benefit much more by being enablers rather than hoarders of content.
My opinion: They don't want ownership of content. They would become responsible for it. They don't want that, because not even they have real control over the data. So you have to confirm in the ToS, that you remain responsible for content you posted, but the system gives you even less control over "your" content than witnesses have. So you are f'ed from both sides. Maximal responsibility, minimal rights. In theory, one single post that somehow contains top secret materials could outlaw the whole blockchain. If you think, well, our witnesses are multinational, just wait until there are some globally enforced laws.