looking back on this you may be right actually, but im not sure if you understood the moral implications of materialism; my point is if the user assumes materialism can be justified they could inadvertently justify nihilism, which cant hold up when you come to the implication value can exist theoretically. so from my perspective it sounds like you're arguing material moral value can exist but that it would have to be in a self justifying way, on which end i would disagree with in its post facto sense.
i would elaborate on this by saying i do believe grounding can be found in the lack of grounding of value itself, as you mentioned with Münchhausen theory, but you would be saying the internal contradiction itself would provide the grounding, which i dont agree with in a material sense, but rather in a post empirical one. the logical implication would not follow as "nihilism assumes nihilism exists, therefore materialism is real" it would follow "nihilism assumes nihilism exist on the basis of materialism, therefore only theoretical value could be applicable"
The indispensability argument does not claim that material moral value exists because of some internal contradiction. If it did, I would agree it is indeed a weak and confused move. The actual point is transcendental, in that certain practices we are already committed to (reasoning, deliberation, explanation) presuppose normative structure. If one participates in those practices at all, he must engage in indispensable deliberation — to even reason about whether materialism is true or not, it requires one to presuppose normative facts about what he ought to believe, thus one is already operating within a normative framework. So the disagreement is not about deriving morality from matter post facto. It’s about what is already presupposed by the activity of giving and asking for reasons.
It should also be noted that materialism does not necessarily entail nihilism, despite it being rather ontologically sparse; there are many problems with materialism, and yes, it often leads to nihilism in many metaphysical concepts; one of the greatest pitfalls of modern society is its highly materialistic philosophy. But, a materialist can still consistently hold that reasons are higher-level facts and that normativity supervenes on natural facts or normative truths are part of the best explanatory framework. Nothing about the mere claim “reality is fundamentally physical” forces the conclusion “there are no reasons or values.” That inference requires additional premises.
Materialism is a metaphysical thesis about what fundamentally exists, while nihilism is a normative thesis about value. As I stated, you need a premise like: “If reality is wholly material, then no normative facts can exist.” But that premise is what I am disputing, since it can’t just be assumed. The indispensability argument is meant to demonstrate that normativity is already built into our rational practices, regardless of one's underlying ontology; it is indispensable to our deliberation, therefore it is prima facie a justified belief to believe in normative realism.
Ultimately, a materialist could hold the position that normative facts are real features of the world that supervene on or are realized by material states, and we are justified in believing in them because they are indispensable to reasoning and deliberation.
You mentioned something about post-facto, so I do want to reiterate it is a transcendental argument;
[1] Deliberation is an unavoidable practice.
[2] Deliberation constitutively involves reasons.
[3] Therefore, participating in deliberation commits one to there being facts about what counts as a reason.
This is similar in structure to how denying truth presupposes truth, or denying logic presupposes logic. The claim is that the very intelligibility of inquiry already operates within normative space. Even to argue that only theoretical value is applicable, you are already operating within normative space, since you are weighing implications and treating some conclusions as ones we ought to accept. These are all normative activities.