I'm not sure these are opposites because of the word "still". Whether he "still" takes over half or he "still" covers half, I don't think there is much difference. "Still takes over half" is probably closer to they way English people would translate this sentence? I'm not a native speaker
What it seems to show me is that "übernehmen" has a continuing sense of "cover", which "take over" does not have. In English you cannot continue to "take over" something, as once you have taken over you are "in charge", "overseeing" or perhaps, "covering." "Cover" can have a temporary sense: When Mrs X retires, Ms Y will take over the department, or responsibility. Three months after this happens, Ms Y is NOT still "taking over", but she could still be "in charge" or "covering". Perhaps in German you continue to take over. I will check this out, as Duo does not always get these things quite right. There ARE people who continually "take over" things, but they are either bossy or acquisitive: "Is Big Company Z still taking over smaller businesses?," or " Is child A still taking over control of the game every time.?" But I presume this is not what Duo means here.
In the context of the sentence, it seems to be a takeover, not a coverage. Context misinterpretation have led to wars, due to bad translations. People who translate these sentences often do not accept comments of any type, from jokes to serior remarks, and often have a out of the box explanation, leaving the issue open for maybe to keepr their point.
That's what dissapoints me about doulingo, having to answer a drill that you are positive it is wrong, but you have to complete the drill set.
I have finished my German course. I'm just doing it for practice. I hope you correct this for future learners.