The aim of this paper is to derive an isothermal Maxwell-Stefan model of diffusion from the moment equations of the Boltzmann system describing a mixture of monatomic and polyatomic gases. A comprehensive diffusion asymptotic analysis of the moment equations, corresponding to an isothermal multi-velocity model of Eulerian fluids, is carried out. In particular, the diffusive scaling of the explicit momentum production term–computed from the Boltzmann collision operator with a cutoff hard-potential-type kernel–enables the explicit determination of Maxwell-Stefan diffusion coefficients in terms of mesoscopic parameters. The model is subsequently validated against the Duncan and Toor experiment as a benchmark, confirming its match with experimental data.