Multiple tab view in Schema browser was able to see both public and private dblink, but the treeview can only see private.
Toad was designed in which multiple tab view can be viewed one schema at a time; therefore, users can see both private and public dblink. In the treeview users can see all schemas; therefore, users can only see private dblink.
To see public dblink in treeview, go to All schema | Public | Db Links.