My understanding is that only Biblical names, such as Jesus and Moses, do not need an apostrophe and an “s” to show possession. Therefore, Jesus’ and Moses’ are correct, but you’d type “The Williams’s house” or “Les’s girlfriend.”
My understanding is that your understanding is mistaken.
The nuns drilled the Oxford comma into my subconscious, as well as: “NO extra “S” for ANY pluralization of ANY word ending in “S”. A simple apostrophe is adequate for the literate.” Sister Anna, St. Anthony’s Elementary ca mid 1960’s.