Collider in RCT Subgroup Analysis

I fully agree with this. If dexamethasone works by: Dexamethasone → ↓Hyperinflammation → ↓Respiratory failure → ↓Death then oxygen need is downstream from inflammation and is endogeneous to the treatment mechanism. Are you saying that the BDGP framing is vindicated in RECOVERY’s design because it achieves causal identification by randomization dissolving the mediator problem? Do you mean that the CIR isn’t clean because oxygen status is a perfect biological proxy but rather It’s clean because randomization makes the control arm a valid counterfactual for each oxygen stratum in the treatment arm and the causal integrity comes from randomization preserving exchangeability, not from the proxy being causally pure?