Откроем английский раздел Википедии на ту же тему, который, как правило, куда полнее русского:
The interest of the system lies in the way it obtains a power supply directly from the single cable on which it is mounted. It is difficult to obtain a very small amount of energy from a line which transports an enormous amount.
Each high voltage cable creates an electric field around itself, in the same way as any live electrical conductor. When the electric potential of such a cable is sufficiently high, its electric field causes a significant voltage difference between the cable and its immediate neighbour. (It is the strength of the electric field that makes it difficult use low powers.)
As a result, a second conductor a few metres long, insulated from but parallel to the high voltage cable, will have a potential different from that of the main cable. Together the conductors make a capacitor charged across an air gap (dielectric).
Under certain conditions, the accumulated charge (and hence the potential difference) is sufficient to trigger a discharge lamp. This what is used in practice, giving a reliable and robust beacon.
Используется то, что поле там очень сильное, и в ближней зоне очень неравномерное по напряженности. За счет этого лампа и горит. Провод - по сути антенна. Ну а емкость - да, емкость есть, но смысл не в этом. Мощное поле от провода гоняет заряды туда-сюда, из провода в антенну, через лампу, отчего та светится.
