It's no different than owning a short barrel for a rifle but not owning the rifle. It's not illegal until it's with it's parent firearm.
Notice their warning about "Constructive Possession". If you own that sear kit AND a Glock handgun, you'd be in "constructive possesion" of an illegal full-auto gun.
Own the kit without the gun, no problem.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.