It turns out MX uses a package called systemd-shim that seems to be the Debian one:

  $aptitude show systemd-shim
  Package: systemd-shim                    
  Version: 10-6
  State: installed
  Automatically installed: no
  Priority: extra
  Section: admin
  Maintainer: Debian QA Group <packages@qa.debian.org>
  Architecture: amd64
  Uncompressed Size: 82.9 k
  Depends: libc6 (>= 2.34), libglib2.0-0 (>= 2.39.4), cgmanager (>= 0.32)
  Suggests: pm-utils
  Conflicts: systemd-shim:i386
  Breaks: systemd (< 209), systemd:i386 (< 209)
  Description: shim for systemd
  This package emulates the systemd function that are required to run the systemd helpers without using the init service