What does “without any disks in use” mean?

• If the computer is powered off, moved or confiscated, there is no data to retrieve.
• We get the operational benefits of having fewer breakable parts. Disks are among the components that break often. Therefore, switching away from them makes our infrastructure more reliable.
• The operational tasks of setting up and upgrading package versions on servers become faster and easier.
• Running the system in RAM does not prevent the possibility of logging. It does however minimise the risk of accidentally storing something that can later be retrieved.
  https://mullvad.net/en/blog/2022/1/12/diskless-infrastructure-beta-system-transparency-stboot/