Ah, I get it!
check-mk-agent will only see the plugin if it is installed in /usr/lib/check_mk_agent/plugins, so the mkp command is not adequate even on the Checkmk server! An extra step is needed.
In the case of the enterprise edition the extra step is that the bakery copies (locally or remotely) there.
In the case of the raw edition some other means (cp/ln/ansible if locally or scp/ansible if remotely) must put in there even on the Checkmk server.
So the mkp command is not really “installing” the plugin; instead it is unpacking it into a staging area, from where one of the two above extra steps still needs to be done.