The origin of the apparent metal-insulator transition in two-dimensional electron systems remains debated. Using temperature-dependent transport and Hall measurements on industry-relevant MoS₂ monolayer FETs with amorphous HfO₂ dielectrics, we establish classical percolation as the underlying mechanism. We report two complementary signatures of disorder-driven conduction onset: a temperature-independent Hall-density collapse as a function of absolute gate voltage and a vanishing activation energy at a characteristic carrier density n_c, identified as the percolation threshold. These findings identify dielectric-interface disorder, rather than channel quality alone, as the key parameter governing device turn-on. The extracted activation barriers reach values substantially exceeding room-temperature thermal energies, indicating that disorder-driven conduction onset remains relevant well beyond cryogenic transport.