After some more code digging and experiments I can answer my own question. A theme is a normal plugin and as such does not inherit any behavior from other plugins.
So you need to copy everything you need from the base theme library.js to your child theme. And of course, you need to declare client scripts explicitly.
Loading of missing templates from base theme is the only link between the base and the child theme.
Doc mistakes the nbbpm property
The documentation states
nbbpm.compatibilityas a property within plugin.json rather than package.json.
I guess this is a bug? Or is the property allowed either place these days?
You're right. We need to have a look at that page, it's starting to get out of date