In single user mode, is the root filesystem not mounted? …
In single user mode, it is (by default) not writeable. Things can be changed with tunefs, and so on.
Bonus: <https://forums.freebsd.org/posts/556568> – education thanks to SirDice
… if you can unmount the filesystem while in multiuser mode, …
Not possible for the root filesystem, but you can:
shutdown now- choose your shell for single user mode.