When Danish has D's that sound like this, they really are D's in terms of what the tongue is doing, they just don't follow through with the sound. That comes out to be similar what an L sounds like, but it's not exactly the same. It's a "soft D". I can't speak for the accuracy of Duolingo's pronunciations, though.
Is this really how the tone would sound in Danish? I just can't imagine anyone saying "En edderkop" like this. "(start) En (down) ed (down) der (up) kop"... If that makes any sense at all xD I would say it more like "(start) En (up) ed (down) der (down) kop". Really didn't know how to portray what I mean, but hopefully some like-minded person will catch on!