Thanks @shinya, that is precisely what I was wondering about, and you have clarified. Any suggestion on improving the documentation? I could submit a PR easily. Was thinking to add clarification to the above-mentioned sections in terms of their interactions with one another (in fact they are mutually exclusive).
Updating this despite the age since it comes up in the top 10 on Google when searching for how to have a job trigger both manually and scheduled. It seems the latest advice is here: Choose when to run jobs | GitLab. Basically it looks like this:
In my case I changed “merge_request_event” to “push” as the former was resulting in “detached” pipelines that wouldn’t run the other non-manual jobs.