"So where is the spark gap in this unit? It turns out to be the ceramic cylinder. I opened up the cylinder and found a stack of eight metal disks with (maybe) carbon electrodes in the center. The disks are separated by mica washers to leave 0.33 mm gaps between each pair. This forms a series of 7 tiny spark gaps.

This type of spark gap is known as a "quenched spark gap". Spark gap transmitters were the first form of radio transmitter, used from 1887 to 1920. They used a spark to transmit Morse code via radio waves (details). The quenched spark gap was one type of spark gap used in these transmitters, as shown in the diagram below.

By combining multiple small gaps, the quenched spark gap could cool off efficiently."

[...]

"Spark gaps generate radio waves across a wide spectrum; inventor David Hughes first noticed this interference in 1878."

[1] https://guelphweddingshop.blogspot.com/

(Although very old, that book is great at explaining the basics of what radio actually is.)