And another thing ...

Here is, perhaps, a better way of wording the argument:

Remember that we are introducing this possibility - that someone may believe that it is not possible to talk - into our present converstaion.  To do this, we need to be able articulate this attribution coherently.

If I say "Mary knows how to talk but doesn't believe that it works", I am either saying something false, or showing that I don't understand what 'talk' means.

The only properly complete answer to "What does 'talk' mean'?" is recursive, and the root is ostensive:  it is what we are doing now, in having this conversation, in writing and reading this post.  And we are doing this successfully (on the whole), or we are not doing it at all.

Since I would be saying of Mary (in the example) that she does not understand this - that she does not accept that we are talking now - I am also saying that she does not mean by 'talk' what we mean by 'talk'; and so that my statement about her does not mean what it initially appears to mean.

It is, in fact, another Moorean 'paradox'.