this article has a better explanation

https://interestingengineering.com/military/submarine-detection-at-light-speed-china

ELF signals, with wavelengths longer than 100 meters (328 feet), typically require large distances between antenna units. Traditionally, generating low-frequency signals needed massive antennas, like the ELF facility in central China, which has antennas over 100 km (62 miles) long.

In contrast, Li’s team has reduced the length of the emitting array to just about 100 meters (328 feet), making it possible to easily install these antennas on Chinese naval ships. The high-frequency, high-power electromagnetic waves emitted by these antennas converge in the sky to create a virtual radio-emitting source. As one source dissipates, another is instantly generated, ensuring a continuous flow of low-frequency signals.

as @D61@hexbear.net pointed out, the virtual source is what's moving at near the speed of light. the virtual source is the sky. the actual source is on a ship. it sounds like they're generating an interference pattern and reflecting that off the atmosphere to produce extremely low frequency electromagnetic waves capable of penetrating the water column. it's the group velocity of the wave packet that's moving at close to the speed of light, or something like that.

I was about to post the same thing ;)

Slightly poorly written article title.

Researchers in China have harnessed the power of the Doppler effect to achieve a breakthrough in submarine detection

According to the researchers, this virtual signal source is capable of emitting electromagnetic waves continuously while travelling at close to the speed of light.

The thing moving at the speed of light is the emitting source.

actually, light speed c is only achieved in a vacuum...

No I had the same thought lol I was like what kind of shitty budget radar waves DON'T operate at light speed?