From the notes section of the article, here were the brands which did not have microplastics:

Only three brands of investigated salts – Taiwan (refined sea salt), China mainland (refined rock salt), France (unrefined sea salt produced by solar evaporation) – did not contain any microplastic particles in both replicate samples. All other salt samples contained microplastic particles.

I was surprised that they found mircoplastics in mined rock salt - makes you wonder how the contamination occured