You misunderstand. My question was why they bothered to make a real "bin" user to have /usr/bin as its homedir; when, if it had just been purely an issue of shoving some binaries over to the /usr drive, they could have just created a /usr/bin owned by root:root and leave it there; or, if they really cared, created a /usr/root for "a secondary root-user homedir that lives on the /usr drive" and then made a /usr/root/bin.

That I don’t know. My guess is that it came later, so that no new user could be called “bin” and thus inherit ownership of those files. There are some historical sources that might shed light on the matter, but someone would have to do some work.