Iran has been supplying the Taleban for years, it's just that catching a member of the Republican Guard doing it would have been significant given their elite status.
I think you misread the situation. If Iran REALLY wanted to arm the Taliban, there'd be a lot more evidence of it, and the US would talk about it a lot more. American officials rarely talk about Iranian involvement with the Taliban and rarely come out and directly say "Iran arms and supports the Taliban". Contrast that with the situation in Iraq a few years ago, when American officials regularly and in no uncertain terms accused the
Revolutionary Guard (not Republican, that's Saddam) and Al Quds Force of arming and training and funding various Shiite militias. And NATO troop deployments along the Iranian border are quite sparse (actions speak louder than words). I mean hell they put the Italians in charge of the area around Herat, which is the nexus of Iranian influence in Afghanistan, and if there's one thing we know about military history, it's don't leave something up to the Italians :rofl:. Alright I jest, but there are only like 2000 or something troops in the areas where Iranian influence is most prevalent, so NATO probably doesn't see stopping weapons coming in from Iran as a priority. Furthermore if Iran really was making a concerted effort to cause American casualties without getting in too much trouble we'd be seeing a lot more Explosively formed penetrator IEDs than they are seeing now in the 'stan. Right now the vast majority are fertilizer-pressure plate bombs from what I understand.
Secondly, don't underestimate how wary Iran is of the Taliban. They are old enemies. In Iraq, Iran had reliable proxies to arm that it could control like dogs on a leash. The Taliban has no intention of being an Iranian dog.
Here's my read on the situation. The Taliban and Iran are wary of each other but they realize that their strategic interest align. Iran also has some very powerful connections within the Afghan government and amongst the old Northern Alliance warlords (Tajiks mostly, the Iranians are always more comfortable dealing with them than with the Pashtun Taliban, Tajiks are basically Persians). Iran wants to cause maximum headaches for the US but still have good options for the post-NATO withdrawal power struggle, so they're maintain some contacts with the Taliban and probably providing them with a few goodies just to keep the relationship open. But I don't think they're arming them at the same level they armed the proxies in Iraq, and I
certainly don't think they're arming them to the level they could if they wanted to go all out.