They're saying the "nearest star other than the Sun", but with some geek-in-group-signalling with the "not" syntax and referring to the Sun as "Sol", which some

They're saying the "nearest star other than the Sun", but with some geek-in-group-signalling with the "not" syntax and referring to the Sun as "Sol", which some science fiction writers (and some non-English languages, of course) use as the Sun's name.