I get that tac exists and is useful for concatenating and printing files in reverse.

Neilk was just humorously pointing out that since cat stands for concatenate, its reverse should be truncate, and since cat is often abused to print a single file to a terminal this fictional trunc should "unprint" it.

Arguably you could view 'comm -3' as the reverse of cat.

  # output is all the lines of file1 and file2
  cat file1 file2

  # output is the lines in file1 not in file 2, and vice verse
  comm -3 file1 file2