It does say in the documentation that you've linked under Referencing Package Members Within Packages that "All references to packages, functions, and classes in the package must use the package name prefix, unless you import the package".
In my opinion, it's a very stupid decision by Mathworks that completely kills the usefulness of packages. Rename the package and you've got to edit every single file within the package that references other files in the package.
It also stop the user of your code to simply wrap it into a package name of his choice (that he knows is not going to conflict with another package name) since the user is then force to edit your code to add references to the package name. So, this just moved the problem of you having to choose a name for your function that is not going to conflict with somebody's else functions to you having to choose a name for a package that is going to conflict with somebody's else package.
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