It’s noncanon. I remember them using it in the novels to explain how “Zefram Cochrane of Alpha Centauri” could be right next door, but by 1996 they decided Cochrane just settled there after developing warp drive in Montana.
Which part of that did you not understand?
Which part of that did you not understand? I forget the exact circumstances, but either they got into a world war, and buried their communication links so that the “other side” wouldn’t be able to intercept any transmissions, or they became afraid that hostile extra-centauri intelligences might overhear them, and so buried their civilization so they wouldn’t be attacked.