They're telling you that if you want to cut down the steer tube past the threads to make it "threadless" you can do it. However, you don't want to do it the other way around- you don't want to thread a threadless steer tube. Most threadless steer tubes are either aluminum or carbon fiber, neither of which lend themselves to threading or clamping forces, or both.
I'm going on a limb and saying that the steer tube that they are using is steel, and if it'll cope with the stresses of a quilled stem and the offset piece in the steer tube, the clamping of a stem won't affect it. And yes, any bike shop that's worth their salt can cut additional threads. It's no big deal, and eventually, the cutter will wear down. But, slide the quill stem in there first and see if it fits. If it does, and it has threads already, then it's suitable for either.
If anyone tells you different, they are wrong, simple as that.