Some are suggesting the US intel came purely from Taliban interrogations. Highly unlikely for several reasons, the starkest being the most obvious: How would the Taliban know who their paymasters were and which specific unit they were from? GRU's tradecraft is sloppy but...

It isn't non-existent. Also, the identities of quite a lot of operatives from 21955 are now known to Western services, thanks in part to their recurring roles in 'wet work' and coups. Look at what @bellingcat has uncovered about them, e.g. how the same Gen. Sergeev who...

...turned up for the Gebrev poisoning in Sofia later went to London to lead the op on Skripal. A much more plausible scenario for how the US linked these bounties to this unit is that members from it physically traveled to Afghanistan and were tracked by CIA/NSA...

from the moment they arrived. If Taliban commanders later captured then said, "We were offered money to kill" and even hazarded that the offerers were Russian (not an unfamiliar nationality to the Afghans), then who these Russians were and which service they were from...

might have been gleaned from pre-existing intel on where they'd been and who they met with in country. In other words, the specificity of the intel here isn't coming from an enemy insurgency; it's coming from we already know about a hyperactive collection of spies and assassins.