Without specifically checking, I'd guess that they probably ignored some of the stuff involved in normal CFD, e.g. pressure-driven flows. In fact, their well-mixed model (their simpler model that didn't need a GPU) basically ignores fluid-dynamics entirely, since it ignores spatial-variation and so there're no fluid-dynamics to model. Their non-mixed model (their more complex model that did use a GPU) probably relied on diffusive-transport, without regard for stuff like pressure-driven flows.

So, yeah, CFD -- lighter on the mechanical-dynamics (like spatial-flows) and heavier on the chemical-dynamics (like chemical-reactions).