The publication manager just displays types with mixin: jmix:publication
or jmix:workflowRulesable
But the "external link" and the "label" did not have one of these mixin types.
The following improvement has been included in the DX 7.1.2.1 hotfix:
https://www.jahia.com/extranet/digital-experience-manager/changelog/changelog-from-digital-experience-manager-v7120-to-digital-experience-manager-v7121
Engines/Managers
[improvement] Added a new jahia.property parameter allowing to specify additional elements like Menu Labels, External and Internal Links, to show up in the Publication Manager ( jahia.publicationManagerNodeTypes )