My issue with this sort of arguement is the mix between no true scotsman and the whole "But that wasn't the actual agenda!" arguements. For the former, it doesn't matter if you don't think they are following the true tenants of Islam, what matters is that THEY do, as far as they're concerned, your not the ones following the true tenants, can argue that point till your blue in the face, but it doesn't change the fact that they believe that they represent true Islam.
Which also leads to the latter, it's an arguement I saw fairly regularly when used to "argue" that Hitler was an Athiest. Could be and he used it to push his agenda, or he genuinely believed his spiel, again, the problem is that what the person at the tops agenda doesn't nessercarily matter in this case, like with the crusades can say "Oh but it's just a land grab yada yada" But the frontline soldiers, the actual armies, they genuinely believed they were doing this, not everyone was in on the "grand scheme" So by treating it as if they were, again will achieve nothing.
Be they true followers of Islam in our opinion or not, they believe they are. So trying to argue that is like trying to argue a no-nuclear missle treaty when your enemy has already launched it's salvo's.