The actual answer is a little more complicated and so doesn't match either of the arguments. While the original card's wording (Italian/English) doesn't match the effect, it doesn't affect played cards (like the Bang! you mentioned), nor does it affect other players outside of their turn (so Bang! cards discarded for Indians are discarded as normal).
This is clarified in their official FAQ on the expansion:
Q4. Where I have to place my played cards while Abandoned Mine is in play, especially when I play Wells Fargo, General Store or Stagecoach?
A. There is a difference between "discarded" and "played" cards: Stagecoach, General Store and Wells Fargo are "played", while cards exceeding hand size limit at the end of the turn or by means of a Duel are "discarded". So, discarded cards goes on the draw deck, while played cards go on the discard deck. Draw the cards after Stagecoach and Wells Fargo always from the draw deck. Note that this applies to current player only: all other players draw and discard normally.
For clarification, here's how it works with certain cards. In all cases, the played card will go to the discard pile:
- Cat Balou: Unless you play it on yourself, the target will discard it normally.
- Duel: The
Bang!cards you discard will be put on the deck, but those discarded by your opponent will go to the discard pile. - Indians: All
Bang!cards discarded by opponents will go to the discard pile.
