Why materialists are predominantly materialists

[Neo-Scholastic]

I'm merely pointing out that, as I understand reductionist models, symbols are seen as structural features of physical processes and their affect. Within that interpretive framework, any significance attributed to a sign system would be a superfluous after-the-fact folk description with no causal role. Am I wrong?

(emphasis mine)
You've gone from criticizing symbols as impotent bearers of meaning to asking about sign systems. Those are very different things. And no I wouldn't say that it is inherent in the reductionist paradigm to assert that [in] a sign system, the symbols play no causal role. It may be a part of some reductionist theories, but I'd say it's more likely to appear as a straw man.

It was a short post so I wasn’t trying to draw out all of semiotics’ subtleties and my inaccuracy led to confusion. The defining concept of reductionism (within philosophy of mind) is that complex mental phenomena are sufficiently explained by simpler and purely physical processes. I say this is problematic for at least two reasons: 1) it confuses signs with their significance and 2) it treats expression and interpretation as entirely bottom-up physical processes. It gets worse too. The reductionist paradigm is itself a cultural artifact that constrains interpretation to fit the mechanistic outlook of modern industrial culture. It’s a vicious circle.