I disagree with your surmise. In a Sengoku Jidai campaign, I believe the Dutch arrive somewhere on the map in 1561, as I stated previously. You may not see them--ever--but that's when they arrive.

In your game I would guess it to be a fluke; you just never saw them. I've seen the Dutch arrive in 1561 exactly, again and again and again when playing Shimazu, Mori, Takeda, Imagawa, etc.. That, combined with documented observations of others on the 'net, convinces me beyond reasonable doubt.

But it doesn't matter ;). Enjoy the game.