"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!
Ugh...your explanation makes sense but I am approaching this from an English speaking perspective so the "OF" wasn't obvious to me. Is it perhaps too much of a generalization that if I can substitute WITH with OF that it's perhaps genitive? Hopefully that makes sense. Bottom line is genitive wasn't obvious to me (it's a struggle in general). ty!
Doporučuji vždy nejdříve hledat na Help for English. Já našel https://www.helpforenglish.cz/article/2015091101-friends-mates-buddies