Deer hair has some buoyancy to it, so it doesn't sink as well as other materials. That reply from the tackle shop mean they don't know, and Yakima means figure it out!
I tie with saddle hackle too, and all saddle hackle & neck hackle are not the same. I used to raise my own roosters, Barred, & White Rocks, to get wide, webby, neck hackles for tying Key's style Tarpon flies, which I was supplying to shops & guides at the time. A lot of the genetically raised chickens are bred to get short stiffer hackles, with very little web, for tying dry flies, and that's not what I wanted for tying the Tarpon & other saltwater flies.
So it really depends on what you want & need. I buy strung neck & saddle hackle, by the pound, and each has different uses for me, but for some tying I can use either one.
I again think that they're using strung neck hackle for the dressing on those spinners, because it is usually a longer fibered material than saddle hackle.