systemd did bring them mainstream, but yea cgroups were around for years before systemd was a thing. But they were mostly independently managed by some small daemons that did things based off rules and other things like that mostly. Systemd changed how they got used, primarily because multiple management systems working with them tends to cause issues, but using them for low impact isolation and tracking of daemons automatically really turned them from something you'd independently setup to something that brings a lot of utility for just about every use-case everywhere.

You can still use cgroups independently with systemd, but it's recommended that you use a service and the delegate option to have systemd create a cgroup hierarchy and also know that it shouldn't mess with it after creating it. This lets you have your cake and eat it too if you have a case that systemd doesn't handle (I'm working on a setup with this myself for a coding sandbox setup).

I think Docker had a bigger influence on cgroup use than systemd.

I disagree because everybody who uses Ubuntu is using systemd, but only a fraction of them use docker.

Ubuntu uses snaps heavily bringing cgroups to play.

I agree with you and edited my wording.