Who's talking about a legally-binding declaration?
Your initial point about him being her knight - which as it turns out he isn't - comes with implications of certain legal or quasi-legal duties coming with the position. "You have to reveal all of your secrets" would be a reasonable stipulation if he were to give any sort of such pledge. But since he has not, the only other way to see this is through lens of Allen-Anastasia interepsonal relationship and Ana's attitude towards it.
Either way, if she decides to one-sidedly claim that "he's hers", and if such claim comes with implications that he has to give up his secrets, and then she sees to it that he does so, well that sounds like a shitty situation to turn up in.
If you get on your knees in front of a woman, then hold a ring up to her, you haven't made any legally binding declarations, but I'll be damned if you didn't send a clear message to anyone watching.
If I gift someone chocolate on 14th of February, would this mean I profess my lifelong love? Or could this mean a mere sign of appreciation, or even a token gesture? Neither we the readers nor Allen know full implications of this gesture, it could be leaving enough room for interpretation, and it observably leaves enough room for Ana to try weasel her way out.
My brother in Christ, the title of the story, and every action he's taken in said story, has been specifically about protecting her. He has dedicated every single breath of his to this one specific woman
In that hypothetical situation where a woman helped me out of my debt, does this mean I am well within my right to demand that she also shares her credit card number and PIN code? Since her actions so far has been about protecting me (from debt collectors) and dedicating herself to me (in form of her labour), this means she's mine, and so she must withhold no secrets from me? She might be in love with me for all I know.
Allen is free to make his own decisions and judgements and protect Ana to the extent he's content with - or rather, that's how it should be. Him dedicating himself (almost) fully to her and of his own volition is a foregone conclusion, and it can be seen in the title as you say. Doesn't mean it entitles Ana to ask for even more