The truth is, we don't know what the main cause of extinction of Steller's sea cow was. There are problems with each of the theories that you read about.

First, these sea cows were massive creatures—they were up to nine meters long and could weigh over ten tons—just enormous. A couple of sea cows could feed a small Siberian village for months and the population of the native Siberian people wasn't very large. So while the Siberians certainly did hunt the sea cows, they didn't need to hunt a lot of them, so it's unlikely they were the ones who brought the sea cows to the point of extinction.

Second, about a hypothetical decrease in kelp caused by ecological disturbances. Well, if something severe really happened in the ecosystem near Bering Island some time before 1768, it would have affected not just the kelp but also other parts of the ecosystem. For example, it would have caused a decline in other marine animals, like whales. But fishing ships in the area did not report a whale decline. Since there's no indication of broader ecosystem problems, the kelp was probably growing just fine, and the sea cows did not experience food shortage.

Third, it might seem like the European traders were responsible because the sea cows became extinct soon after the Europeans arrived. But actually, by the time the Europeans arrived, the sea cow population was already quite small. We have evidence that the sea cow population was at its largest hundreds of years before the 1700s. So something was causing a serious and ongoing decrease in the sea cow population long before Europeans arrived in the Bering Island area. Whatever this “something” was, it should be considered the main cause of the extinction, not the European traders who were just the last to arrive.