Okay, but you must see that in principle it's perfectly possible for "lo" to be referential yet for "broda" to be part of the claim. Referentiality doesn't entail the description not being claimed.
In English, nonrestrictive relatives have independent assertive force, which is preserved even if the relative is within, say, a question or a command. If you think Lojban "noi" works that way, then I accept your reasoning, merely noting that there are other viable candidate meanings for "noi" (such as the one I had been thinking it had) that would invalidate your reasoning. (It's possible to have nonrestrictiveness without the independent illocutionary force, and I had supposed that in the absence of any specification of independent illocutionary force, noi is merely nonrestrictive.)
1: I (hereby) ask whether it is dinner time yet.
2. The reason for my asking whether it is dinner time yet is that I am hungry.
I (hereby) state that the reason for my asking whether it is dinner time yet is that I am hungry.
By my thinking, (1) consists of an illocutionary operator "I hereby ask wh", with 'propositional content' "it is dinner time yet". So if (2) is "the reason for X is that I am hungry", X is not the propositional content of (1) but rather is (1) itself, i.e. the illocution. Can you reexplain where exactly you differ?