After Quebec the official way is via Taxiway A Northbound. It usually isn't critical and you will often find that local crews will ask for Bravo because it is a smoother surface than Alpha.
You will find that when it IS critical (usually when landing on 36C) ATC will, when giving a clearance to cross Quebec, re-iterate that the aircraft must route via Alpha with the statement "After Quebec, route Alpha." All aircraft taxiing in will be via Bravo and told to "Hold abeam A26" to ensure that no-one goes head to head on Quebec, which is the only bottleneck at Schiphol.
It is a great shame this happened. It is a bit confusing in this area, made even more so when the airport realigned the runway entrances a couple of years ago. Yes, we all know that the Dutch are an arrogant race ("If you're not Dutch, you're not much" is a popular statement within KLM) but this could have happened to anyone. But really it shouldn't happen at your home base and I am sure that is what the investigation will focus on- how did crew co-ordination break down to such an extent that this happened. Possibly, they were being cool and blase about the departure, but I suspect they won't be making that mistake again in a hurry!
ATC have to shoulder some of the blame here as well- I have been to the SPL tower on several occasions and it is absolutely state of the art up there, with displays of GMR etc. in front of the controller. I think he or she will be involved in the de-brief!