Reasoning
The Giddings-Mangano safety case excludes dangerous stable black holes because cosmic-ray-produced ones would have destroyed observed low-magnetic-field white dwarfs within their measured ages. But in the metastable (scenario-3) case, applying G-M’s own section-7 machinery with the same illustrative parameters (L = 10^-7 m, M_N = 1.9x10^5 kg) - a mBH Eddington-accreting at the centre of a white dwarf radiates ~5.9x10^19 W ~ 1.5x10^-7 L_sun, roughly 10^4 times below the measured cooling luminosities of the G-M white-dwarf sample, hence undetectable; and its accretion timescale for the star exceeds the white dwarfs’ ages by a factor > 10^10. Neutron stars are similarly unspectacular. So the continued existence of billion-year-old white dwarfs and neutron stars carries no information against this scenario: a star could have harboured an Eddington-limited mBH since its formation. G-M’s published bounds on Eddington-limited accretion were derived for their scenario 2 with Hawking radiation switched off, and therefore do not apply to Hawking-radiation-limited accretion. The general lesson drawn: ‘events which are catastrophic for Earth must also be [catastrophic] for compact stellar objects’ is an intuition, not a theorem - it fails exactly where the danger comes from steady radiation rather than consumption.
Step 6 verdict
Verdict: approved (checked). Reconstruction: (P1, scenario premise) a metastable mBH inside a white dwarf accretes at the Eddington limit with the stated illustrative parameters (L = 1e-7 m, M_N = 1.9e5 kg); (P2) the G-M white-dwarf sample has measured cooling luminosities ~1e-3..1e-1 L_sun. (C1) L ~ 5.9e19 W ~ 1.5e-7 L_sun, ~1e4 below cooling luminosities → undetectable; (C2) star-consumption time exceeds WD ages by > 1e10 → WD/NS survival carries no evidence against the scenario; hence the astrophysical exclusion does not reach H-8. Conditional on P1 the arithmetic and the evidential-screening conclusion follow, and the scope claim (G-M’s Eddington-limit bounds were derived for scenario 2 with Hawking radiation off) is a fair reading. The real dispute is P1 itself - A-53’s photon-trapping/advection physics contests whether Eddington-limited radiating accretion can operate inside degenerate matter at all - but that is premise truth for steps 7-8, not an inference failure here. Same conditional-validity standard as applied to the G-M chain. Valid.