Can you point me to one serious postmodern theorist that holds this view? There are many different views of the postmodern, but rejection of meta-narratives (academic speak for "grand narrative" here) is pretty much universally accepted.
Given how closely this description of post-modernism is associated with Lyotard, one can only surmise that other post-modern authors were, if anything, even less consistent with it.
These are all small micro-master narratives which each have a totalizing perspective and collectively reduce the ‘truth’ to the ‘truth that is out there’ in the intro sequence to the X Files.
Furthermore I would point to his book On Seduction which is is even more accelerationist than Simulacra in that he tells you exactly how to tear the roof down.
I don't understand why postmodernism is described like this, when the typical presentation of postmodern ideas (even, and perhaps especially, in the primary sources) is literally phrased as a grand narrative, albeit one that points towards a forcefully stated skeptical outlook where even "complex meanings" are ultimately unknowable. Isn't the whole thing a little self-defeating at that point? There are more and less meaningful ways of "tearing down assumptions", and the postmodern approach just tends to come with a lot of theoretical baggage (much of it essentially tacked on from modernism itself) that ultimately weighs it down.
I'm not sure if metamodernism fixes this issue, but your description of it suggests it does not. Philosophical pragmatism ultimately seems to do a better job of shedding the "grand narrative" tendency, and that tradition is largely independent from postmodernism.