5.3 All propositions are results of truth-operations on elementary propositions.

Truth-operations are the way that truth-functions are produced from elementary propositions.

In accordance with the essence of truth-operations, a new one arises from truth-functions in the same way that their truth-functions arise from elementary propositions. Every truth-operation begets from truth-functions of elementary propositions another truth-function of elementary propositions, a proposition. The result of every truth-operation on the results of truth-operations on elementary propositions is again the result of a single truth-operation on elementary propositions.

Every proposition is the result of truth-operations on elementary propositions.

Clear? Or should I re-translate?

So the whole machinery here rests on elementary propositions. And wasn’t that concept called into doubt above? See 4.221.