From my understanding he didn’t create any sort of new officially sanctioned union. He simply allowed blessings of same sex couples with some caveats. Mostly that the blessing cannot occur directly in connection with any sort of union ceremony (marriage, civil union, etc) and that it cannot involve any of the typical trappings of a marriage ceremony so as to avoid confusion.
So basically, you can’t get the blessing at your actual wedding. You can’t get the blessing if you show up in tuxedos/bridal dresses, exchange rings, whatever else even if technically it isn’t a wedding. Simply show up together on any random day as a couple and ask for a blessing without those things? Sure they’ll do it. Whereas before they couldn’t.