Interesting. I got two responses on my phone marked as spam, while here on my desktop they are nowhere to be seen - presumably they went straight to my spam box. Before that, I got a German language message which apparently has to do with upgrading tickets.
It may well be that the apt repo is not bundled. That is why I asked the question. But the github repo readme includes this language:
pgAdmin 4 will be bundled with the EDB PostgreSQL 9.6.1 installers,
pgAdmin III is bundled with the Windows installer,
So I think it was, and remains, a reasonable question, but since the one answer I have was marked as spam for some reason, I'm not sure I should trust it. Any other input, please?