The rice of the sea: how a tiny grain could change the way humanity eats
I do have some experience here... interestingly there is an ongoing push to cultivate seaweed around the world, mostly in East Asia but also some in Europe. It's worth a few billion a year now annually, with Indonesia as a big player. Think ropes covered in the stuff, on giant plantation grids, looked after by small teams of employees in boats and a few divers.
Seaweed has some potential: It doesn't require extensive inputs or scarce land, grows well if you have a naturally fertile area, produces a crop reasonably high in protein or other useful extracts and could be good for maritime ecosystems and carbon sequestering...
However, currently it's largely grown for high value extracts (thickening agents and other useful products) as if you're growing for protein you have to price against soybeans, and good luck with that at current prices (what drives crop prices is actually quite interesting, but medium run assume cost per hectare divided by yields multiplied by a narrow profit margin of like 5-10%, which makes soy very cost effective). This also rules out growing for carbon (there's barely a market for sequestering) and almost certainly growing for carbs like in the story above.
3.5 tonnes per hectare also could be a bit high based on biomass in other seaweed crops, but I don't know the eelgrass specifically... it's not impossible, but it might be a case of scaling the small manicured test plot to a full hectare and assuming yields are the same. They won't be, and going to a million hectares would be worse.
As a result, it's veyr very unlikely to be competitive with rice in normal circumstances unless you're selling to fancy high end restaurants looking for something fun for the menu.
Seaweed could still be useful for food security in bad disasters, mostly as its shocks aren't correlated with land crops (drought is hard in the sea for example). However, this isn't the current driver of day to day famine around the world, world food prices are still low by historic levels and like Marinuso said below most serious enduring famine is a man made crisis these days. If the world food programme can access your area you'll get to eat, even if crops fail, but if militias are blocking the roads, you starve.
Basically then, this seems to be a toy for high end restaurants, and patrons who want to think they've returned to something more eco, or pure, rather than any serious change in how humanity will eat.