Sorry, can't agree. I believe it says American B-17s specifically to distinguish them from B-17s in RAF Bomber Ccommand squadrons. The RAF first used B-17s as early as 1941:
RAF Bomber Command - Timeline of events
and kept using them, with 100 Group, right to the end of the war:
BC - Group Stats
Maybe you are right, but I have never heard of the Americans joining the RAF in their night raids, I have on the other hand heard of the RAF using B17's from Coastal Command during the early 1000 bomber raids and also to raid French ports. I expect somebody on here can help us with this one, I am feeling to lazy to check it myself.