"Linux ISOs" is just anything pirated in general.
It really was common in the early 2000s for Linux distros to provide torrent downloads. Internet speeds were such that downloading peer‐to‐peer could be significantly faster, and (probably more importantly) it saved the distro mirrors a lot of bandwidth. I think this practice has mostly died out among newer distros, but distros from the era such as Ubuntu or Arch still provide them.