Fair enough if we're constraining the discussion to just "plastics in the environment" instead of "impact to the environment."
My point is that simple dichotomies are often too blunt to be helpful. As an example, silk is a natural fiber. Is it better for the environment? Many would say no because of the relatively high environmental impact compared to synthetic silks.
Suffice to say, the way the wording framed it as "natural" vs. "plastic" is probably less useful as a general heuristic.
We're saying in the case of clothes, natural means breathing less plastic and less plastic going in to our water.