My first thought seeing this was "seaweed sequesters carbon; harvesting it isn't carbon neutral and doing so at mass scale would do enormous damage"

But I decided to read the article anyway and was initially glad to see they addressed this in the first half: describing the carbon sequestering properties.

They then proceed to completely ignore that in the second half of the article, and provide no explanation of how harvesting it at scale would not be disastrous.

AFAIK, seaweed farming at scale is the best know way to remove carbon from the ocean - which is desperately needed to reverse acidification. Farming seaweed for food is good, but you have to build the market a lot too. Cattle feed and biofuels would be great drivers of scale for this.