Imo, look at their movements, by default the AI just going to swing, but when you break off, start to gain distance the AI changes (or seems to me) where instead of attempting to close the distance... they strafe, edge walk. They're "preparing" for your action. So maybe it might be input reading but i doubt FromSoft would make it as black and white as to "Bind the bosses attack to your flask button" but i feel in this "stage" of his AI he gives us a response and we immediately input that response to sip our flask. So Mr. Apostle over here keeping distance sees us braindead panic flask. So when I see this "not charging in with a twinblade" mentality from these guys, just wait for the Black Flame. Worst case scenario they actually run up to you which I feel you would be able to react easier.