Dune external-lib-deps does not list all required libraries

I use dune external-lib-deps to check what libraries a given project needs as rpm BuildRequires. For qtest commit 074da5e3a0d480937ddc379fc6f4feeab9da85e3 the output looks like this:

+ dune external-lib-deps @install
These are the external library dependencies in the default context:
- bytes
+ dune external-lib-deps @runtest
+ dune external-lib-deps --missing @install
+ dune build @install
File "src/dune", line 18, characters 23-29:
18 |      (runner_libraries qcheck oUnit bytes)
                            ^^^^^^
Error: Library "qcheck" not found.
Hint: try: dune external-lib-deps --missing @install

Why does dune not report the runner_libraries?