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Could anyone knowledgeable about waves and physics related to it shed some light into possible practical implications?

This just provides the means for accurately measuring frequencies in the THz range.

This is important because previously it was possible to measure accurately either lower frequencies, until a few hundred GHz, or higher frequencies, from infrared to ultraviolet, i.e. from tens of THz to hundreds of THz.

Measuring accurately frequencies allows accurate spectroscopy in this frequency range, which can provide information about the chemical composition of materials.

'Frequency combs" are devices that can do at much higher frequencies what digital frequency dividers can do at low frequencies, and this frequency division function is what allows frequency measurement, by converting the high-frequency signals into low-frequency signals, whose frequency can be measured with the classic methods.

The method described in the article has the additional advantage of high sensitivity, i.e. it can measure very weak signals, which could not be distinguished from noise by less sensitive methods.

There are natural astronomical sources, from blackbody radiation. It doesn't penetrate the atomosphere so space based observations only.

TFA mentions spectroscopy and non-destructive scanning instead of X-rays.

Rydberg atoms aren't antennas. When modulated and then read out by the electrical field of a laser they can be used to infer the ambient electrical field at a arbitrary frequency over very, very narrow frequency bandwidths. This can be used to receive radio signals. But it's not very good at it and it's not an antenna. While the specific frequency can be tuned over a very large range the instantaneous bandwidth is still too narrow to actually receive anything but narrowband carrier (no modulation wings) and barely that.

These are physics tools for specific things, not general radio receivers for transmitted information.

Yes, what is really needed is a way to baseband a good swath of spectrum, e.g. in the neighborhood of 100MHz to 10GHz, so that conventional electronics can be used to study something more than simple low rate there/not-there activity. And conversely a way to modulate similar bandwidth onto arbitrary frequencies up in the THz region.

What's an antenna if not something that can receive radio signals?

The same way an LED is not a solar panel. It will give you some voltage, but basically a rounding error above zero.

Antenna are about capturing energy over macro scale areas. This atom is measuring electromagnetic oscillation at a particular point in space. Technically you can recover a signal, but only a rounding error above the noise floor. It doesn't capture energy.

> What's an antenna if not something that can receive radio signals?

>> the instantaneous bandwidth is still too narrow to actually receive anything but narrowband carrier

so it is a "useless" antenna.

Could it pick up a THz chirp signal?

https://en.wikipedia.org/wiki/Frequency_comb