Does this only mean that the man is at the computer and possibly not using it, or does this mean he is using the computer? In Dutch the literally translated phrase "the man sits behind the computer" would mean he's using it, so I translated this with "the man is always using the computer"
Technically it can mean that he's only sitting next to it not using it, but you'd need a particular context for anyone to actually interpret it as such. "Sitta vid datorn" is basically a fixed expression with the sense of "to be at the computer, to use the computer". If you wanted to be less specific you could say "han är vid datorn", which would often imply the same thing, but usually not as strongly, and would be more possible to interpret as a mere statement of his location.
Oh, haha, I didn't use that answer, I translated this with "the man is always using the computer". I was just asking if it's similar to the Dutch expression. It's not correct in English, and I don't think it should be accepted as a correct answer. After all the man is not literally sitting behind a computer. :)
But is that the point though? I have to learn via the english version of Duolingo because there are no swedish courses in my mother tongue. So in order to learn, I have to translate everything in English and I don't always know the idiomatic way of saying it in English, which does not mean that I did not understand the Swedish sentence. Thus, I think Duolingo should always accept many possible translations in English even though they are not 100% idiomatic.
Very often it appears i am incorrect due to the wrong choice of English words in translation or the wrong word order. I am not the native but still happy that i got the right meaning as i am learning properly. But 'at the computer' would be the last thing to come on my mind as the possible idiom