Imagine that I said to you "The only sentence with any meaning is the one you are reading now, and it means exactly what it says."
This has to be false, a kind of nonsense. Apart from anything else, it excludes the possibility of any statement explaining the meaning of the 'only meaningful statement' having any meaning.
Why does this incoherence disappear when we are dealing with a whole linguistic practice?
For instance, "Only statements which are part of practices we recognise as language use have any meaning, and they mean exactly what they say" (or some suitably qualified equivalent) must be literally true.
There are some differences between these statements, though:
The first refers to a closed group - it has only one member. The second is not closed - what counts as a 'statement which is part of our linguistic practice' is not determinate. This can, in fact, be discussed and adjudicated upon within the linguistic practice. (It can only be discussed and adjudicated upon within the practice.)
What is involved in a statement 'meaning exactly what it says' is not unambiguous. (Remember the 'Brexit means Brexit' sleight of hand). It's like the joke about a balloonist who lands in a field in the fog, and asks a passer-by where they are, getting the reply: "You're in a field".
We can only have an unrestricted capacity to make meta-linguistic adjudications (about meaning, truth, etc.) in an 'open' language - this is exactly why formal languages are always so disappointing. Not to say useless for any sort of serious enquiry ...
Crucial enquiries nearly always have a semantic element - incorporate some (tacit or explicit) investigation of meanings. Whether or not some specific statement 'means exactly what it says' is always open to interrogation.
