Sorry but I don't buy this. One needs to ask a very basic question - How would US transport ISIS fighters to A-Stan?
There are only three ways it can do it. Via Iran airspace, Via Central Asian/Russian Airspace, and via Pakistan Airspace.
Unless Iran is a Traitor, I don't see US using Iran airspace.
Central Asian/Russian airspace. Unless Russia is a hypocrite and has 'zero' control over Central Asia, this option is not possible as well.
Third option is via Pakistan airspace. Unless Pakistan is a total Retar.d to deliberately allow US to transport ISIS fighters to A-stan border so that these same ISIS fighters would then attack Pakistan, I don't see this option happening as well.
There is also Chinese airspace but this option is close to impossible.
In conclusion, except for a few dozen ISIS slipping through the cracks, presence of foreign ISIS fighters in A-stan is all hot air, and full of BS. The only so called ISIS fighters in A-Stan are "Locals" trained by the US. And these so called local IS fighters are being "taken care of" by the Taliban.