Yes, clearly you didn't understand what you read.
It's not limited to IPv6 or systemd.
The link you gave has the same lists of affected distros which includes
Devuan (sysV init)
MX Linux 19 (Mepis+antiX)
Void Linux (runit)
Slackware 14.2 (rc.d)
All non-systemd distros. Also, again in the link you supplied which I think is the actual team that found this, they are using all IPv4 in their commands, and even state this at the beginning:
So it was discovered on IPv4 first.However, we recently discovered that the attack also works against IPv6, so turning reverse path filtering on isn't a reasonable solution, but this was how we discovered that the attack worked on Linux.
That being said, it seems like a very, very tricky thing to do and I doubt this is a thing to be concerned about seeing in the wild.