The sub-Neptune K2-18b has become the most discussed object in the search for life beyond the Solar System. JWST transmission spectroscopy of its hydrogen-rich atmosphere has yielded firm detections of methane and carbon dioxide and a contested, repeatedly re-examined signal attributed to dimethyl sulfide — a molecule that on Earth is produced almost exclusively by marine life. Phosphine, a second gas long proposed as a biosignature, belongs to the same assessment problem. This article does not attempt to adjudicate whether the dimethyl sulfide signal is statistically real; independent reanalyses disagree, and that disagreement is treated here as data rather than as a question to be settled. Instead, the article advances a conceptual argument that holds whichever way the statistics fall. Drawing on fifteen Scopus-indexed studies and seven institutional sources, I introduce and develop the principle of detection–biogenicity decoupling: on a hydrogen-rich sub-Neptune, the atmospheric properties that maximize the spectroscopic detectability of a candidate biosignature molecule — low mean molecular weight, an extended scale height, a large transit signal, and a reducing photochemistry — are the same properties that maximize its abiotic production and accumulation. Detectability and biogenic diagnosticity are therefore not independent; they are structurally anti-correlated. The very features that make K2- 18b an efficient JWST/NIRSpec target make it an inefficient biosignature target. The principle is supported by three lines of evidence: the spectral degeneracy of the candidate molecules with abiotically plausible species such as ethane in the NIRSpec range; the demonstrated abiotic syn- thesis of dimethyl sulfide in cometary and interstellar environments entirely devoid of life; and the photochemical plausibility of organosulfur and phosphorus chemistry in a hydrogen-domi- nated envelope. The implication is that a NIRSpec detection, however statistically robust, cannot by itself constitute a biosignature on a hydrogen-rich world, and that biosignature assessment must shift from the detection of a molecule to the demonstration that its abundance exceeds the abiotic ceiling for its specific planetary context.