So far so good.
Not exactly. While this effect happens, what is actually happening is they are making the antenna electrically a 3/4 wave antenna. 3/4 wavelength is resonant. The resonant point of an antenna is also the low impedance point of that antenna, so in a round about way the effect you see happening is the same. However, very rarely is the best point to tune these, or any antenna for that matter, at 50 ohms of impedance.
The radials are a radiating part of the antenna. They are the other half of the antenna. Each half of the antenna does affect each other so yes, they will affect the feed point impedance. They also affect the radiation pattern of the vertical radiator. There is far more going on with them than simply to help with tuning.
Not being directly familiar with them I took a look. The piece you are referring to has the same effect as the ring in your Workman, however, they have something else attached to both the radiator and the ground plane portion of their antennas. As I am not sure what this does I don't know what affect it will have on their tuning.
Up to a point (just past the 5/8 wavelength point) as you lengthen an antenna it radiates less and less RF energy up, redirecting that energy out. This can lower the radiation angle, but it is also putting more RF energy into that lower lobe. That lobe will also be more sensitive on reception as well so it will affect your receive the same as it does your transmit.
And yet they perform better higher up, at least locally. If DX is your goal a number of other factors come into play.
The DB
