Learn how to write a command that limits searches to org-mode headlines

I use imenu, but only headlines with no subtree are shown

I'd recommend using `org-ql-find` for this, as this is its designed purpose. It's fast and "smart", and compatible with built-in Emacs completion, as well as Vertico.

I'm not a user of imenu so I can't help you. I don't know how it does the selection.

If I'm not wrong `org-ql-find` works using the Emacs completion in the minibuffer. And the point of the video is to have a command with the search experience of Isearch, that means without using any completion at all. Another benefit is to not add dependency on other packages for a simple command. But I keep your command in mind for other use cases.

Thanks for the clarification.