I think you have not quite got the right hold on the conception of truth that
Foucault has in mind here. The key is in Foucault's talking of truth's being 'produced'. We'd normally say that truth is discovered or discoverable - not 'produced', which suggests that truth or what passes for truth is manufactured. Truth, or perhaps we should say 'truth', on Foucault's account is the product of systems of social power; and each system produces its own variant of truth. If power and freedom are opposed - freedom is freedom from power - then the truth is not liberating if it has precisely been produced by (a system of) power. Truth is something akin to the products of Marx's 'false consciousness' in which beliefs (at least the bourgeoisie's) are pure results of class position - taken to be true but actually not true at all.
'Systems of power' is a vague phrase but what's more important here is to see that, for Foucault, at least in regard to your quote, truth is (in a phrase he didn't use) a social construction. 'Truth' as a product of social power is only a simulacrum of truth.
See the final section of 'The Political Function of the Intellectual' from which your quotation comes : https://www.scribd.com/doc/22531213/Foucault-The-Political-Function-of-the-Intellectual.
What does Derrida hold ? His subtle views are easily caricatured and I cannot do proper justice to them here but I might say that 'the problem of truth' for Derrida is connected with the undecidability of language. It is not the case that, as Saussure thought, in speaking or writing we simply give expression to our thoughts and communicate them to others. What we say or write has a shifting meaning : we say or write in one context but what survives, our reported speech or a text, can exist and function in quite other contexts from which we are totally absent (perhaps dead). From those contexts any intentions we had in expressing and communicating are absent too. In different context it acquires different meanings.
Any attempt to specify what Plato or Kant meant either in a whole text or even in a single sentence is irretrievably lost. Neither the text nor the sentence becomes meaningless but it has no single, fixed, timeless, context-free meaning.
▻ FOUCAULT AND LIBERATION THROUGH MEDITATION
Your question is centrally about Foucault. I think his view of truth as a function or product of systems of social power principally relates to the discourse of science and to everyday discourse. He should not need to deny that some buffer can be created against social power under the special conditions of meditation. In fact he recognises that there can be freedom in the interstices of systems of social power, in the gaps between them. In such a gap meditation and liberation can occur. Indeed, did his own work not proceed from his own occupation of such a gap ?