As the statistician George Box famously said of theories (aka "models")....That's true in so far as we can't test all the circumstances, but the limitations of a theory does not necessarily make it false. We still use Newtonian Mechanics as it works for understanding the Orbits of most of the planet of the solar system. the exception is Mercury which doesn't orbit in quite the way Newtonian mechanics would suggest, but that was later explianed when Einstein theorised was affected by the sun's gravity bending space-time. Even thought it does hold for the one example, doesn't mean it loses its validity in all the contexts where it does accurately reflect what happens. So, if materialism was "true" it would be true in a sense relative to our ability to discover and reproduce phenemeona in accordance with our ideas rather than being absolutely true.
"...all models are wrong, but some are useful."