There is no fixed level in inferno mode to get a unique item Any level can give you a unique cause it drops randomly. A good tip is to play bosses rounds (like 1-10, 2-10, 3-10 & etc.) cause they drop multiple items per round and will give you a better chance of acquiring one.
My friend Sohan got a inferno sword on playing level 1-10 and another friend in 9-10. Also other players who posted they got theirs.. possibly. We also got lots of unique armors and accessories that now it's making my inventory full. So it's legit there are drops. Just last week I got an inferno shoulder unique playing inferno mode 2-10 level. I got a lucky drop there by using my lightning unique set over and over 😙