Causes do cancel out in some structures, and Nature does not select randomly (e.g. evolution might select for cancellation for homeostasis reasons). So the argument that most models are faithful is not always convincing.

This is a real issue, a causal version of a related issue in statistics where two types of statistical dependence cancel out such that there is a conditional independence in the data, but underlying phenomena are related.

I don't think gwern has a mistaken epistemology, however, because this issue exists. The issue just makes causal (and statistical) inference harder.