Actually, this was revealed in late 2001/early 2002, but the Bush Administration quickly shut it up since it held some delusions that the Iranians could be enlisted to help topple the Taliban and fight Al Qaeda. I remember the stories well, which went right down the memory hole overnight.
The rationale was that Iran DID back the Northern Alliance for a while against the Taliban, but it clearly was an alliance of convenience since Iran didn’t want Pakistan to become a larger player in the region. Iran could care less about the Northern Alliance and a stable Afghan government, and anyone that gets in their way is just seen as an obstacle.