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Well a passport is a bit more than just a travel document, as it certifies someone's citizenship, and without that it would basically appear to leave them stateless. And, as you say, that statement is primarily aimed at keeping people in the UK, not preventing their return.
I saw an interview with the Attorney General, or a former one, saying we can not make her stateless under both UK & international law, and we can't stop her returning. Meanwhile in Parliament yesterday, Javid finally admitted the same, saying - While the UK cannot leave people stateless, under international law, he said any such Britons would be "questioned, investigated and potentially prosecuted".
That doesn't mean the UK has go out of it's way to provide help, and she would appear in limbo, being in Syria with no British Consulate or Embassy available, but if she manages to cross the border and get to one, they will have to issue a passport or some other legal travel documents.