Using the Green-Bogoliubov method, we derive an expression for the thermodynamic pressure or grand potential density for a molecular model of a fluid confined in a disordered porous matrix. The expression is equivalent to the virial equation derived previously via the replica method [Rosinberg et al., J. Chem. Phys. 100, 5172 (1994)] but differs from an expression recently derived from the condition of mechanical equilibrium. It is concluded that the condition of mechanical equilibrium does not yield information about the thermodynamics of these systems. The thermodynamic pressure can be obtained from experiment indirectly by integrating the Gibbs adsorption isotherm and, in principle, by direct measurement.