Who says monoliths don't have clear and explicit separation of concern between domain concerns? I think that just comes down to how the codebase is organized and how disciplined the team is, or possibly breaking out core parts into separate libraries or other similar strategies - technically it's still a monolith.
Libraries are a great way to manage separation of concerns. Any dependency you add has to be explicit. There's nothing stopping you from adding that dependency but you can't just do it accidentally.
The graph of dependencies between components makes for explicit separation of concerns just like you would have a graph of dependencies between different network services.
Or you just use a language with support for clear module API boundaries (vs something like Ruby where without bolt on hacks, every piece of code can call any other in the same process).
