Do you have a link to the video? I'm wondering if you have caught the pattern of emphasis correctly.
I'm wondering if the context could have been something like this: "These tiny copepods live by eating the algae they swim among. So do some of the largest animals. Only, the largest animals _dive_ for _their_ algae."
In other words, "only" is a linking word meaning "the same in one way, but different in another way," rather than meaning "this is something that is only possible for the largest animals."