§50

The standard metre in Paris is neither one metre long nor not one metre long. Why say this? - Because it plays the role of being the standard by which we determine whether other things are one metre or not (or by which we calibrate metre sticks). You might say that 'the standard metre is one metre long is true' - but if we can say that it is true then it is not true in the same way that descriptions/empirical claims are.

Wittgenstein uses the standard metre case to make a point about the metaphysics of the Tractatus. Just as we cannot say that the standard metre is either one metre long or not one metre long, we can attribute neither being nor non-being to the elements. This is because by definition within the system 'being' and 'non-being' consist in the obtaining and non-obtaining of connections between elements. In the standard metre case the standard metre plays a normative function - it is a standard by which metre sticks are calibrated. Similarly, in the elements/destruction case - it is a grammatical rule that destruction = something obtaining/not-obtaining between elements - and so an element on its own cannot (according to the rule) be destroyed.

It's a mistake to think of this as some kind of metaphysical oddity.