The only thing I can think of is that in different regions, this hay may go by different names, if you know what I mean.
Usually the only thing I've ever heard of salt marsh hay being used for in this area is by contractors and road crews to be kept in bales or spread in areas to keep down any and all growth. Or to block runoff water from an area.
I've simply never heard or remember anyone using it in their garden.
I sincerely hope that it turns out super for you and your garden. I would love to be wrong and have learned something new with this.