On the “Reposettings” page, if the GitHub integration isn’t enabled for the repo, you get an error message saying:
GitHub Integration is installed. However, this repository is not enabled.
The former is only available to organization admins, while the latter allows any org user to request that the app be installed for that repo (this flow allows the org admins to approve the request)
edit: note that this is when adding codecov to a repo owned by an organization, not a personal repo. If the second link format was used, then the link could consistently always link to the same spot – just the installation id would be required