The study presents three scenarios assuming different assumptions about the mass of plastic particles. In the worst case scenario, where I think the assumptions are bad, they estimate 5g of plastic per week. In the other two scenarios which used more complex but more realistic modelling they estimate 0.15 and 0.3g of plastic consumed per week so only a credit card sized amount per year.
In these scenarios (0.15g and 0.3g) about 90% they estimate comes from salt.