Like saying oranges and cucumbers aren't "truly" the same thing. Or apples and carrots. Or tomatoes and lettuce. They are all edible plants, the same thing. On the other hand, some are fruits and some are vegetables. Wait, lettuce and carrots are vegetables but lettuce is a leaf and carrots are a root. "The same thing" depends on the definition of the categorization. Nothing is "truly" the same thing. Everything is unique in being a unique permutation of atoms and molecules making nothing, "truly" the same thing, except by a categorization that is envisioned by people. No marriage is exactly the same thing, or "truly" the same thing. On the other hand, if the category is "marriage" than SSM and HSM are exactly the same thing because they are "marriage".
You are creating your own categories, an artificial definition that you like.