After some time investigating, it seems that my understanding, as expressed in the above comment, was not correct.
The extra reference v18.0.0 is required, not by the Jenkins scripts themselves, but by lsstsw implementation logic.
In fact, lsstsw rebuild of lsst_distrib 18.0.0 will fail if not specified the extra reference v18.0.0.
From a release management perspective, 18.0.0 release will build only with v18.0.0 version of DM Externals.
Having this degree of freedom is error-prone and makes the process very weak.
The 18.0.0 release is identified by two tags, and both are needed in order to build properly the release.
This is also confusing from an end-user point of view.
I hope that the external packages will be moved to conda soon, as suggested in DMTN-110, in this way we can speed-up the Science Pipelines release process and make it more robust.
I suggest we consider this issue closed as “Won’t Fix”, as soon as I can restart the 18.1.0 release process.