From the article: You are thinking a lot of strange thoughts when you are sitting with an oxygen mask on in a smoke filled aircraft (quote from one of the passengers).
And: There was so much smoke that all the passengers had to put on the oxygen masks...
My guess is they may have used the masks as a means to calm down the passengers.
A spokesman from KLM downplayed the incident, saying, yes, there was smoke in the cabin, but the aircraft returned safely to AMS.
No info as to the type of 737.