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.
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.
* https://lists.freedesktop.org/archives/systemd-devel/2016-Fe...
Lennart Poettering is indeed now pushing for systemd to impose a similar requirement for /var .
* https://lists.freedesktop.org/archives/systemd-devel/2016-Fe...
* https://lists.freedesktop.org/archives/systemd-devel/2016-Fe...