Silly, silly, silly me. The reason the pglogical files are there are because I ran
sudo apt-get install postgresql-16-pglogical
to put them there. (It was a long day of DevOps yesterday.) Running a dpkg -L postgresql-16-pglogical pointed that out for me…
Running
sudo apt-get install postgresql-13-pglogical
got pglogical up and running for me no problem. I am going to keep exploring this further, but at least I am unblocked on my urgent task.
However, I think the point I make above about the versions in discourse_docker and postgres remains.