Ockham on semantic independence

In chapter 43 of Summa Logicae, on the properties of substances, Ockham gives a curious argument to support the nominalist view that truth and falsity are not real properties of propositions.  He begins with Aristotle's discussion of contraries, and of the puzzle that the very same proposition (i.e. declarative sentence) can admit of both truth and falsity.  The sentence 'you are sitting' is now true, for you are sitting. But then you stand up, and the sentence is false.  But the sentence hasn't changed.  How can it both be true and false?

Ockham argues that the truth and falsity of propositions are not some sort of quality inhering in them.   Otherwise, it would follow that a proposition was truly altered by the fact that a fly was flying.  And certain heresies would also follow - and here comes the curious argument (my translation).

For if the truth and falsity of propositions are qualities of propositions as whiteness and blackness are qualities of bodies, then whenever some truth exists, ‘this truth exists’ will be true, just as whenever some whiteness exists, ‘this whiteness exists’ will be true. And in the same way of any falsity. Then I accept the falsity of the proposition “God newly creates something”, which according to that opinion is a single quality of the proposition, inhering in it, and as a consequence is something other than God.

Then I ask whether that thing can be created by God, or not. If it cannot, then it is something other than God which cannot be created by God, which is against the Evangelist, who says “All things were made by him: and without him was made nothing that was made.” [John 1.3]. But if it can be newly created by God, let it be given. Then “this falsity is newly created by God” will be true, and “this falsity is newly created by God, therefore something is newly created” will follow, and further “therefore it is true that something is newly created by God”, and as a consequence it is not false, and furthermore “therefore this falsity of the proposition does not exist”, and further still “therefore it is not newly created by God”.