Translation:They enter and sit down on the sofa.
Well, if you are inside watching them come in, you can see them sit on the sofa. If you are outside watching them go in, how do you know they are sitting on the sofa? I just used the word "enter". So, "and sit on the sofa." is what implies that you are probably already inside.
Yes there are two complete sets of forms. https://www.thoughtco.com/asseoir-to-seat-to-sit-set-down-1369835
Yes, it is wrong. If you were outside watching them go in, then you wouldn’t see them sit down on the couch, so you must be inside already watching them enter and then you would say “come in”. “enter” has the advantage of being able to be used from either perspective.
Authors narrate information that's not visible from a single viewpoint all the time, as do people formulating plans, as do people involved in gaming. Whether or not to use "go" or "come" is a translation issue, not some sort of requirement that every sentence involve only a single, fixed viewpoint from within the setting of the narrative (because no such rule exists).
after reading all he comments, all I can do is quote Charlie Brown, "Good Grief!" Maybe it would be better if Duolingo divided itself into two programs--one for native English speakers, attempting to learn French, run by bi-lingual anglophones, and one specifically for francophones attempting to learn English. Then maybe we wouldn't get so tangled up in each others' linguistic and grammatical oddities. Again, may I suggest a special lesson or lessons on idioms and some of the (to Anglophones) peculiarities and apparent inconsistencies of everyday French..