They are “undeclared” in the sense that the Buddha never declared them as true statements. That doesn’t mean they aren’t false. In fact, it’s because they are based on wrong assumptions (an existent Tathagata-entity before death) that the Buddha never declares them as truth. That’s why he said “they don’t apply” in MN72 and SN44.1.
Other religious figures kept insisting that one of the four must be true. It’s kind of implicit in their logic. One of the four has to be true: “A”, “not A”, “A and not A”, or “neither A nor not A”. You have to declare one of them as truth.
But the Buddha said, well, the whole underlying assumption “A” (i.e. an existent Self) is a mistake. He said ‘Tathagata’ is all without Self, therefore he didn’t declare any of these four as truth. Because all four imply a Self in one way or the other. (SN44.8)
In the suttas they are never said to be “neither true or false”. They are just undeclared. Which is different. So the Buddha didn’t declare them, because with the assumption of Tathagata as a Self, they are in fact all four false.
I hope this clarifies it. 