-fvisibility=hidden only affects the linker visibility, i.e. whether symbols are visible when a linker tries to link against your file. It does not specify any active obfuscation.

Your strings are still placed inside a data section and need to be loaded into the memory space of the process when your library is loaded, so they will still need to be visible when inspecting the file. If you need them to be obfuscated, you will need to obfuscate them yourself and decode them at runtime (knowing that a sufficiently determined attacker can still reverse engineer or debug your library).