"I am friends with those women."
Translation:Jsem přítel tamtěch žen.
The question and answer directly above your question indicate that a "to be friends with" verb-plus-instrumental case construction can be used. But... the instrumental case hasn't been introduced at this point in the course.
So the main translation at the top of the discussion uses a simpler "to be" verb-plus-genitive construction instead. In this construction, the Czech sentence is literally "I am a friend OF those women," and that's where the genitive comes into play.
I hope this makes sense!
It would have been a lot easier to translate into Czech if the English had been, "I am a friend of those women." Thanks BoneheadBass.
I wonder whether it should be "I am a friend..." rather than "I am friends..."
Yes, "I am a friend of those women" would make for an easier and more direct translation. Perhaps a change will be considered for the next version of the course.
It does look weird, but the phrase is very often used, at least in the US.