why arcgis online item already share to public, but not visible to public is because this item owner is portal, portal does not share to public, even arcgis online it share to public.
This tells you that this item data source is NOT in arcgis online, arcgis online can't control its access level. Instead, it says this item data source is enterprise portal, you need to go to portal to share it to public.
From data source URL, I can confirm that this item is not from arcgis online, it is from enterpise portal.
all item from portal is like gis1.victorvilleca.gov.....
all item from arcgis online is like xxxx.arcgis.com........
