A pendulum is just energy storage.
The amount of energy it stores at any given time is given by
mgh +1/2 mv^2
where m is the mass of the pendulum, h is its altitude above the low point of its swing, v is its velocity, and g is acceleration due to gravity.
That's it. There isn't any more energy in that pendulum than any other pendulum of that length and mass in that gravitational field.
The amount of time that a particular amount of energy can do work is determined by the rate at which it is used, or the power.
Apparently, it's a very low friction device that doesn't use much power.
Why would you find that astounding?
In terms of craftsmanship, it may be quite an achievement. As far as the physics goes, it's trivial.