Four inches shallower, which means four inches less depth of water. That’s what “shallower” means. Listen, if the satellite is measuring the location of the buoy relative to some elevation datum, that’s fine. Makes sense to do it that way. But the story is talking about the water column, which to me means depth of water. If the writer means something else, he or she should say so.
Your logic reminds me of a carpenter my dad worked with. A piece of wood was too long and he would mark an end. My Dad would say, why not cut this end its closer and we don’t need to turn the board. The guy would answer, but this is the end that’s too long.
Anyway...a story came out and debunked this story. So we are safe.