What Popup brings up is the first part of the Free Electron Gas model. One of the next steps in that model is realizing that the gas will respond to different frequencies in different ways. How the medium responds to frequencies is known as the dielectric function which depends on two things. One is the frequency of the EM wave. The other is the plasma frequency which is a constant for a given gas - it depends on the electron concentration, charge of the electron, and the mass of the electron (with a fundamental constant or two thrown in for good measure). What happens when the EM wave hits the medium will depend on what the wave’s frequency is in relation to the plasma frequency of the medium.
The periodic table is arraged such that elements in a column have the same outer electron structure. It’s this outer structure that corresponds to the electron concentration for our ‘gas’. All of the alkali metals have the same outer structure (and one that makes it darn easy for people to use in models!) and thus have the same plasma frequency. The plasma frequency for these metals and the frequencies of UV light relate such as to allow transmission.
I’m not sure if that was any of an explanation or just repeating the statement with more words.