I'd have to watch the episode again, but from my understanding, it had to do with the frequency of the energy being absorbed by the creature.
All energy has a wavelength, as you can see in a chart of the Electromagnetic Spectrum.
the 21cm wavelength in real life has something to do with the wavelength of energy detected from hydrogen gas in interstellar space.
Not sure how that ties into the episode. I'm actually curious to watch it now.