There seems to be documentation issue with Microsoft Defender ATP for Linux. The system requirements, as far as I can see, do not mention that systemd is needed. I found this on a Debian 9 (Stretch) system that was configured with SysV init. The post-install script of mdatp performs some tests that use the systemctl command, which is of course missing without systemd.

Update: Microsoft has meanwhile confirmed that systemd is indeed required.