Phrases of the form "if you must ... " (or indeed "if you must!" without a following verb) are a particular idiom in English.
It uses "must" in the sense of obligation, but the idiomatic part is that it is implying "you think you must do this, but I don't agree, and I'm going to provide or permit whatever you are asking, but grudgingly".
Edit: actually, it can be used even where there's no suggestion that the other person thinks that they 'must', but just that they're doing it anyway "If you must use that word, please don't do it in front of me!"
But in the case of "if you must know", since their knowing is dependent on the speaker's telling them, the implication is indeed "if (in your opinion) you have to know".