The coupled coils approach is so old school I'd suspect is had origins in the late
20s or 30s if not before with spark. It persists because it is easy to understand,
build and does indeed work. It also offers selectivity as its a band pass filter. That in
a high BC radio environment might be very helpful.
They are of course local to the antenna as any form of transmission line tends to
make a simple life, complicated. The upside is even at power they can be fairly compact.
It can be done at the bottom of a half wave vertical. As far as tuning remotely, it could work
to the limits of the variable cap at a harmonic of the base antenna. But do keep in mind a
full wave or longer 1.5 wave pattern is not the usual broadside sine-cosine pattern. For a 1.5
wave vertical at 28 mhz the peak power is at something like 75 degrees, not very useful.
For a half wave it will be quite low more like 21 degrees, very useful.
The whole thing of compromise antennas is that for one this its that and for another its a solution.
Which is which is the exercise of experience.