NIGHT GUES
No it is not true because i got inferno sword by playing same stage
It's definitely not true bro because what happened to me is a different one. I got more uniques playing old levels than newer ones and sometimes getting 4 different uniques in a span of 24hrs of hard grinding when I'm in your level. Maybe that's why you got a inferno sword. There is no pattern.. it just drops when it's your time to get one.
Also you can get dark items in inferno mode in late stages (8->10) if you're looking for some. I got some items but very rare. You can have more chances of getting a wing in the shop than in drops. Nanoo said it's possible but haven't known anyone did.