For Touchstone, although I doubt he'd put it quite so plainly, the only truth is the truth that there is no absolute truth (or at least there is no access to absolute truth).
However, he's not consistent in his belief. He himself believes he has good reasons to believe this, sure, but a significant problem arises when others disagree with him.
That is to say, when others disagree with him, Touchstone doesn't simply shrug his shoulders and sigh, "Well, it's too bad they don't understand what I'm saying. But it's not their fault. Or anyone's fault for that matter. In fact, no 'fault' comes into play at all. I mean, I honestly can't blame them or anyone else for misunderstanding me, since I believe we live in a world in which there is no truth as such, in which words only have relative meanings, relative to a person's own biases and experiences and so forth, and in which the very act of communication itself is suspect. So, what can I say? Honestly, I can't say anything at all, really. Neither against anyone or for anyone. It's just what it is."
Nope, that's not how Touchstone replies. Instead, he gets all worked up, and responds in a huff, word-for-word, line-by-line, and even paragraph-by-paragraph to everything that's been said about him and his arguments! He challenges Steve's, or Peter's, or Gene's, or Paul's, or my or whoever else's words, claiming that we have misunderstood what he said when he said such and such, to such and such, or about such and such, or regarding such and such, or pretty much about whatever else comes to mind. Touchstone gets all riled up and goes to great lengths to defend himself by responding to others, heaping words upon words until he feels he's had his say. He quotes chapter and verse, so to speak, of his own arguments and/or the arguments of others. He informs us how and why we must've misunderstood him here or there when he said what he said. Or how we must've misrepresented him in this or that. In fact, I suppose there's a good chance Touchstone will respond in the combox of this very post and so prove the point again.
But a few of the assumptions Touchstone makes are that communication and understanding are possible, that there are objective statements and truths, and that some truth is able to be successfully communicated. Otherwise, why bother to try to communicate in the first place? Why work at it, why work at trying to get one's point across, as he clearly does? Why try to respond to others? Why try to correct others' misapprehensions or mischaracterizations or what not of a particular position?
The simple answer is because, contrary to what he professes to believe, Touchstone behaves as if things like truth and objectivity do exist.