It's a badly worded subtitle, probably by an editor. From later in the article: "By measuring light of precisely the needed wavelength — peaking at precisely 21.106114053 centimeters".

it is not 21 in binary number system nor in mayan, so does not matter, it is just number. same importance for physics as horoscopes, numerology ...

but i was pretty annoyed after i read in article - "exactly 21 cm" and then inside of first diagram - "v=1.4GHz" ...