If a single environmental source seeds an outbreak, the earliest cases are enriched for direct exposure to that source; as person-to-person chains lengthen, successive generations are increasingly infected away from the source, so the fraction of cases with direct source exposure declines monotonically over calendar time. The observed drop from 55% (pre-1 Jan) to 8.6% (from 1 Jan) matches this expectation for the Huanan market as the seed. The inference is valid only conditional on unbiased exposure ascertainment; it does not by itself distinguish a true market source from an ascertainment artifact (see the competing ascertainment argument), so it raises but does not settle the market-epicenter hypothesis.
Step 6 verdict — approved (checked)
Reconstruction. Premises: (i) a point source seeds the outbreak; (ii) transmission then proceeds by lengthening person-to-person chains; (iii) ascertainment of source-exposure is unbiased. Load-bearing step: under (i)-(iii), the fraction of cases with direct source exposure declines monotonically over calendar time, so an observed 55% → 8.6% drop is the expected signature of (and therefore consistent with) a market seed.
Evaluation. The generative logic is sound: generation-0/1 cases are enriched for direct source contact, later generations are increasingly infected away from the source, so the direct-exposure fraction falls. The obvious undercutting defeater — the same decline is produced by ascertainment that preferentially found market-linked cases early (see A-18) — is not denied but explicitly fenced off: the statement claims only “consistent with (though not proof of)” and conditions on unbiased ascertainment. So the weaker, non-decisive conclusion the argument actually asserts is immune to that defeater; nothing further needs correcting. Traced myself; author-blind.