That's not what I remember. Wikipedia backs up that recollection as well [0], marking systemd's initial release in 2010. Docker's initial release is listed as 2013 [1]. Maybe that was true of the dotCloud internal releases, and certainly, not all of the EL and other Linux distros had not adopted systemd during 2010 - 2013. Certainly after 2014 or 2015, systemd had spread to the major Linux distros so Docker could have chosen to take a systemd-based approach at that point.

[0] https://en.wikipedia.org/wiki/Systemd

[1] https://en.wikipedia.org/wiki/Docker_(software)