Thanks for those details.
Its not that convoluted. IMO ISIS-K taking advantage of having the world's attention to the Kabul airport they attacked the Americans and have thus score a tremendous propaganda win.
Think about it, the whole world now knows about them. They are taking over the "anti-US" role from other groups and have demonstrated their ability to harm US troops
Plus if they managed to convince the US to stay in Afghanistan then that would give them a further recruiting boost in Afghanistan and in the wider region.
No I don;t doubt what you say in the least.
It is simply that to have such a specific warning on the specific day, means that the West knew it was coming and where it was coming from. A simple way for this to be true is to have given bad news to a no longer required asset and to have received a very real threat back by return!
ISIS can surely launch a simply random suicide bomber attack in total surprise. This has been a known and appreciated risk from day one of the evacuation. This attack was known about and very precise.