The modern animals that dwell along the beaches and then go swimming (seals, sealions, walruses, whatnot) are carnivores; they enter the water to hunt fish or shellfish. There isn't a whole lot of grazing available that close to shore. So, we have two data points for the animal being a carnivore. In the actual published paper (not the article above) odds are the researchers will say the animal was probably a carnivore.
It's built on supposition. That's what we're telling you. And you can believe it because you're smarter than the rest of us.