For me the comparison to monorepo made a lot sense. One of the main features of monorepo is maintaining a DAG of dependencies and use that to decide which tests to run given a code change. CRAN package publishing seems to follow same idea.
No, that's the opposite of a monorepo (w/continuous integration). A monorepo w/continuous integration does not maintain any list of dependencies or relationships, by design. Every single commit is one global "version" which represents everything inside the repo. Everything in the repo at that commit, is only guaranteed to work with everything else in the repo in that commit. You use continuous integration (w/quality gates) to ensure this, by not allowing merges which could possibly break anything if merged.
Maintaining a DAG of dependencies is a version pinning strategy, the opposite of the continuous integration version-less method. It is intended for external dependencies that do not exist in the current repository - which is why it's used for multi-repos, not monorepos.
But as I originally pointed out, you can have a monorepo where everything is version-pinned (not using continuous integration). It's just not the usual example.
Each component within the monorepo will declare which other components it depends on. When a change occurs, the CI system figures out which components have changed, and then runs tests/build/etc for those components and all their dependencies. That way, you don't need to build the world every time, you just rebuild the specific parts that might have changed.
I think that specific concept (maintaining a single "world" repository but only rebuilding the parts that have changed in each iteration) is what the author is talking about here. It doesn't have to be done via a monorepo, but it's a very common feature in larger monorepos and I found the analogy helpful here.
What a monorepo gives you on top of that is that you can change the dependents in the same PR