The Gentoo news post is not about having /bin and /usr/bin as separate directories, which continues to work well to this day (I should know, since that's the setup I have). That configuration is still supported.
The cited post is about having /bin and /usr on separate partitions without using an iniramfs, which is no longer guaranteed to work and had already been awfully iffy for a while before January. Basically, Gentoo is no longer jumping through hoops to make sure that certain files land outside /usr, because it was an awful lot of work to support a very rare configuration.
I don’t know if this is really a “so broken” instance. /bin and /usr/bin (or sbin) have never been well separated, to the point where many distributions just symlink to /usr anyway. If you don’t want an initramfs to provide binaries you need them somewhere accessible.
They were defined sure, but without distribution adherence they weren’t actually, this has been the case for a long time. Out of all the distributions, Gentoo is probably one of the most sensitive to this issue since most others have used initramfs or initrd for decades and Gentoo has always made it optional.
If the post was about FHS adherence I’d agree more.
Think about booting over network. Or having /usr on another drive. Including even network drive. Think about dumb terminals(wrong cetury) thin clients. For example they can use small disk to quickly boot wihout downloading kernel and initramfs and use NFS for /usr and /home.
It exists because, long ago in a galaxy far far away, a sysadmin ran out of space on a drive. The system was split between two 10MB(?) drives, one was / and one was mounted at /usr, for User data. They moved some of the programs to a folder for a dummy user, /usr/bin, and put that in everybody's PATH.
Everybody kept on doing things that way ever since. Social momentum is funny that way.