What about microplastic shedding when washed?
So it appears that this is not a plastic. Maybe you could check to see if this fibre is actually biodegradable?
I think it's a valid question that its status as not being petroleum derived does not necessarily answer meaningfully.
“Once discarded, chitosan-constructed objects are biodegradable and non-toxic.[66] … Pigmented chitosan objects can be recycled,… Unlike other plant-based bioplastics (e.g. cellulose, starch)”