Weird how nobody noticed it until now. It is a leftover from a long list of songs. Players could see song IDs in the server panel (we had one few years ago) and they could play that song by specifying its ID in /play. That was removed because I was the one that had to add songs there. It was a good decision to keep only /radio, as I'm not the one that's playing the music for them. I couldn't keep the list updated with new songs, while it is a radio's job to provide all kind of songs, even the newest ones.
Thanks for reporting.