= Solution
At fixed reactant partial pressures and temperature, their ideal-gas bulk concentrations $C_i=p_i/(RT)$ remain approximately fixed. Lowering total pressure means removing diluent rather than automatically lowering the reactant concentration.
In the continuum molecular-diffusion regime, gas diffusivities scale approximately as $D\propto1/P_{\rm total}$. Delivery through a comparable boundary layer is therefore faster, increasing $h_m$ relative to $k_s$. \b[The system tends toward reaction-limited operation], where concentration differences between substrate locations are smaller. This commonly improves thickness uniformity and access to recessed surfaces.
The qualification is important: changing pressure can also change flow velocity and the boundary layer, gas-phase chemistry and precursor depletion. In narrow structures at low pressure, Knudsen transport limits the simple $1/P$ improvement. Improved transport does not correct a large substrate-temperature gradient in a strongly temperature-sensitive reaction-limited process.
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