I think they kind of cover that:
> They found the chemical biomarkers of seaweed and aquatic plants in 26 samples, which suggests that early humans were eating—or, at the very least, chewing—these bounties from the sea
Like, yeah maybe the fish they were eating was covered with seaweed, but unless they were popping the stomach/intestines into their mouth... for the parasite load(?), the seaweed in the fish is digested. This isn't Carbon-14 dating, they identified families of plants on the the ancient teeth, not digested plant matter.