Diamond has a rather elaborate concept of causation which differentiates between ultimate and proximate causes. On the pain of oversimplification, ultimate causes are those material conditions that make life possible and sustainable, primary of them being the availability of sufficient food to sustain a population. Proximate causes, otoh, are cultural adaptations to the availability of resources needed to sustain life. The organization of economy (i.e. capitalism) and society would be a proximate, but not an ultimate cause in this scheme.
Moreover, his concept of causation goes beyond simple deterministic schemes of the being-determines-consciousness variety. Although he never uses the phrase, the relationship between ultimate and proximate causes involves "path dependence" which can be defined as a progressively increasing probability that an initially adopted solution to a technical problem will be replicated in subsequent iterations of solving that problem. The mechanism that sustains path dependence is transaction cost - lower to adapt an already existing solution than to come up with a brand new one. Path dependence is one of the key concept of institutional economics that directly undermines the "law" of diminishing utility which is at the core of the neo-classical economic theory.
In Diamond's causal model, path dependence is what explains a collapse of a civilization. An initially successful "solution" to the problem of procuring resources necessary to sustain a population becomes ingrained in the cultural fabric of that population and becomes increasingly difficult to change, even if the conditions that originally made that cultural adaptation a success change.
However, a true genius of Diamond's analysis comes in his ability to procure counterfactual evidence to prove his relationships - he focuses on geographically isolated societies that share the common social roots to examine theoretically relevant differences while maintaining the ceteris paribus condition.
PS. I agree with Chris that Proyect seems incapable of understanding any complexity that goes beyond real or imagined personal motives of the people involved. Therefore, personal attacks are his favourite and only modus operandi. If his ranting were published anywhere, it would be the National Enquirer of the left.
Wojtek