This is how I understood the argument: Parts can’t form machines which perceive (I couldn’t find a justification for this assumption – that’s the sticking point), therefore perception is not created by parts but by a substance.
The meat comment targets the first part of the argument. Leibniz seems to make an assumption (parts can’t from perceiving machines ≈ meat can’t think) which he doesn’t really justify.
I thought he was using the mill as an example to show that perceiving machines made from parts are self-evidently absurd, to ridicule the idea of perceiving (or conscious) machines. What am I missing?