The systemd maintainers were one of the first ones to put together a technical proposal and do a lot of the implementation work for the UsrMerge, in a way that was reusable across distros.
systemd itself does not depend on being on a UsrMerge'd system, and otherwise, the proposal does not have anything to do with systemd.
You are quoting 2013 doco. systemd has (again) changed since then.
As of 2016, the position is that whilst there is still code in some of the program to handle a split /usr, a significant part of the system (in particular Plug and Play device management) now references /usr and depends from it, to the extent that it is already a requirement that /usr be always present: i.e. that it either be on the root volume or be mounted by /init (on the initramfs) before it invokes systemd.
Uh, I don't see how this is related to UsrMerge? You can still have separate /usr/bin and /bin directories, you just need to make sure /usr is mounted and accessible during boot. Requiring /usr be available during boot has long been the case for Linux, even before systemd came along.
Iirc, glibc used to be maintained by a RH guy until he got on so many people's wrong side there was fork made that ran for a number of years until the original was merged into it and the fork renamed.
systemd itself does not depend on being on a UsrMerge'd system, and otherwise, the proposal does not have anything to do with systemd.
From https://www.freedesktop.org/wiki/Software/systemd/TheCaseFor...
"Note that this page discusses a topic that is actually independent of systemd."