Not the only one, but very important. In addition, it should be borne in mind that the killer whales are spread almost all over the planet, from the Arctic Ocean to the Mediterranean Sea.
In addition, killer whales share the territory in Antarctica with another apex predator - sea leopard, according to one of estimates, their population reaches 440,000.
No other ocean has such a density of higher predators. And each such predator needs a certain area for hunting (by the way, we can roughly estimate it).
Firstly, the density of the other predators isn't what is important. It is how much food they take from the food chain.
You are also ignoring another predator, humans, which take loads of fish out of the ocean.
Humans eat roughly 20 kg of fish per person per year. That amounts to roughly 140 000 000 000 kg.
This significantly reduces the amount of food available for the orcas.
However, humans typically don't eat penguins, and don't go fishing too far south.
This makes Antarctica a good place for the orcas.
Yes, there are also sea leopards, and other species of seals which compete for the penguins, while themselves being eaten by the orcas.
Seals are not apex predators, it is the prey of orcas.
What does this have to do with the FE?
I was assuming it was an attempt to claim the southern ocean must be much larger than the globe Earth claims.