Gentoo also did it from the get go, although it's not SysV.
Except you don't know really know that. It is objectively true that such usage was around for a long time before systemd in some major distros. I was seeing it at least 10 years ago, and I still see it on my systems that don't use systemd.
> the green [ OK ], is what many people already identify systemd with
is true for people like me that were not familiar with those other systems.
For example, most probably use Ubuntu (and probably only Ubuntu), and as far as I know it didn't display those [ OK ] until after it adopted systemd. So, at least there's a very large portion of users that never saw those [ OK ] until systemd came along.
If I remember right, it was the same case with Archlinux, which is where I was first exposed to systemd.
Imagine, the biggest news in your distro is the migration to systemd and the first and most obvious change you notice is that the boot process has all these neat [ OK ]s instead of random text. I think it's to be expected that people would identify systemd with that.
That's an interesting comment, since the green [ OK ] was in use for bootup messages long before systemd was a thing. Nothing about it says "systemd" to me.