I don't get it? They seem to be arguing in favor of bootc over systemd because bootc supports both split /usr and /usr merge? But systemd is the same. There is really nothing in systemd that requires it one way or another even in the linked post about systemd it says:

Note that this page discusses a topic that is actually independent of systemd. systemd supports both systems with split and with merged /usr, and the /usr merge also makes sense for systemd-less systems.

I don't really get his points for it either. Basically boils down to they don't like mutable root filesystem becuase the symlinks are so load bearing... but most distros before use merge had writable /bin anyway and nothing is stopping you from mounting the root fs as read-only in a usr merge distro.

And their main argument /opt and similar don't follow /usr merge as well as things like docker. But /opt is just a dumping ground for things that don't fir the file hierarchy and docker containers you can do what you want - like any package really nothing needs to follow the unix filesystem hierarchy. I don't get what any of that has to do with bootc nor /usr merge at all.