Last sentence is correct- as for NN, if you do not see it, it is 100% NOT in that bird in any way or form. It's a single dominant gene. You can breed a bird with naked neck with any other bird, barred rock, polish etc and the chicks will come up with naked necks.
NN has nothing to do with scaleless. Completely separate genes. Scaleless actually showed up in a flock of New Hampshires, no naked neck involved. Scaleless is recessive which means a bird can carry it and you can't tell by looking at it... but there is a little something unusual, some carriers do show areas of missing scales on their legs but otherwise they look normal.
Since then, scaleless has been mixed up with NN though, mostly for the fun of it. A NN bird carrying scaleless looks the same as any other NN, except for few of the carriers showing areas of missing scales on their legs.