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.