Although I haven't considered this passage for a long time, I think it is the flip side of his later view about "sense" (Sinn, meaning) and truth — that is, a proposition is something that can be true only because it could be false, and vice versa. In other words, for a string of signs to have sense or meaning in a sign system, it must be possible for it to be either true or false. Or, it must make sense whether it is either true or false. Or, a false statement has meaning just as much as a true statement has meaning.

At the same time, a tautology — something that "must be true" according to the usual definition — is not really a proposition, in the sense that it doesn't actually tell us anything about the world, and therefore isn't really "true." That is, if it can't be false, then it also can't be true. Instead, it refers to a "rule of grammar." For example, the law of identity, "a = a," is a tautology and not a proposition. It does not assert anything. Instead, according to the later Wittgenstein, it is an insignificant corollary of the grammatical rule that allows two equivalent strings of signs to be substituted for one another. If we observe that rule, then or course any sign can be substituted for itself. It's not a useful application of the grammatical rule, but it is consistent with it.

This later view is flipped 180 degrees from his earlier perspective about the relationship between propositions and tautologies. In his Tractarian view, this quoted passage is a corollary of his perspective at the time that every true proposition is a tautology. Therefore, any normative statement (for instance, about ethics or aesthetics) can't really be asserted, because it is not grounded in "the world" or "facts." So, as the passage suggests, anything about which there is only doubt, because it is not grounded in the tautologous relationship between factual experience and truth, cannot really be expressed (in language). Basically, the passage plays around with the complexity associated with legitimate doubt versus illegitimate doubt, which arises when one assumes that all true propositions can be "reduced" to tautologies, and all false propositions to contradictions.

I think what connects the two points of view is Wittgenstein's insistence that the "connection between language and reality" really determines what can be asserted or "said." And because he feels some sort of duty to have absolute integrity with whatever his viewpoint on language was at any particular time, he struggled to express what kind of "limit" around language his theoretical viewpoints implied. So in his early work, he ends up writing these somewhat mysterious-sounding statements about what can be asked, what can be said, what lies beyond the limits of the world and therefore propositional assertion, etc.

In his later work, he generally ends up declaring certain things he disagreed with to be "nonsense." There's a tendency to view these declarations as just provocative, but I think they were literal extrapolations of his evolving thoughts about "sense" or meaning. If a string of signs exemplified nonsense, he meant that it really had no meaning or sense, according to his perspective of what sense really is, how language really has meaning, etc. So if a philosopher (including especially his earlier self) expressed a point of view that was "nonsense," he meant — this way of thinking doesn't capture the way in which language appears to work in all its instances, it's too one-sided, etc. Because his later work "grounded" language in sense rather than truth, his view on non-sense became likewise more elaborate.