Does the 2nd and 3rd sentences not read right or is confusing to anyone else?
I don't see anything wrong or confusing in them.
On running the merge-usr script, the contents of /lib will be moved to /usr/lib, and /lib will become a symlink to /usr/lib. The same will happen with /bin (merged with /usr/bin) and /sbin (merged with /usr/sbin).
/usr/bin and /usr/sbin will remain separate directories as they are today (and as /bin and /sbin are today). Some other distros have also merged /usr/sbin into /usr/bin, but Alpine is not doing that for now, as it's still being discussed by the FHS.