Janai no

Hi! In that example, does it say "it was their choice (wasn't it)" or " it was not their choice" (what they chose to do)?

(人) が選んだことじゃないの!

To me, it sounds like "it was their choice", but some translations say otherwise and it confuses me.

Does it depend on the context?

Thanks.

Yes, it totally depends on the context. Grammatically, positive and negative are both possible for "noun (こと in your example) + じゃない（の）" (intonations are different in spoken conversations, though). As for "verb/adjective + じゃない（の）", じゃない（の） is a sentence final particle, and always works as positive (not negating the preceding word).
e.g.
選んだじゃない（の） ＝ 選んだ
選んでなかったじゃない（の） ＝ 選んでなかった
高いじゃない（の） ＝ 高い
高くないじゃない（の） ＝ 高くない

Incidentally, （人）が選んだことじゃないの！ as "It was not their choice!" is often used in the context "It's my(or someone other than the mentioned person) choice!".

Thank you very much for the explanation!