Yet just by mentioning the (very widely held) notion that these problems do have such a basis

The issue isn't that you mentioned the notion that the problem might be due to faulty neuroendocrine function but that you assume that talking can't do anything about that.

I'm apparently espousing dualism. It's very strange.

If you limit talk therapy to the goal of changing the mind and ignore hardware than you lose effectiveness.

Granted it's impossible to get good feedback to do targeted interventions on the biochemical level but the body is still vitally important.

But even given SSRI isn't targeted intervention on the biochemical level. According to a recent article:

It is currently impossible to measure exactly how the brain is releasing and using serotonin, the researchers write, because there is no safe way to measure it in a living human brain.

Instead, scientists must rely on measuring evidence about levels of serotonin that the brain has already metabolized, and by extrapolating from studies using animals.

The best available evidence appears to show that there is more serotonin being released and used during depressive episodes, not less, the authors say.

SSRI might also work by reducing inflammation. They also hit targets outside the brain. Depression correlates with inflammatory cytokines. There are efforts underway to focus on diagnosing depression with blood tests and if those tests come the prime measuring stick the official definition of depression might even include inflamation.