Following the instructions in the above-linked article, I need to
create a PolicyDefinitions folder under
\\domain-controller\SYSVOL\Policies
and then drop in the ADMX files.
According to this comment, you did not have a Central Policy Store before (which is perfectly legit, because it doesn't exist by default and you need to create it manually).
When you create the CPS, you must populate it; you do that by copying into it all the default ADMX templates for the most recent Windows version. You can get them either from the most up-to-date Windows computer in your environment, or (better) by downloading them from here.
After the CPS is created and populated with the default templates, you can add your own ones; they are available for lots of applications, including Edge and Office.
What you did wrong was creating a CPS and populating it only with the Edge templates; when you created the CPS, all GPMCs in the domain switched from using the local templates to those in the CPS... but your CPS only contains the templates for Edge, thus all the default ones are missing.
Solution: add the base ADMX templates to your CPS, and everything should work correctly.
In reply to your comment: yes, if you use the CPS, you need to update it manually whenever a new set of ADMX templates is released, either for Windows or for any additional application.