Again, they are not explicitly allowing synthetics in general. They made one oil for the Renesis that they are calling "synthetic," but that doesn't mean all synthetics are okay. Heck, we don't even know what Mazda's oil is made of and how much is synthetic. It could be mostly dino with a little bit of syn added. There are far too many unknowns to say whether all synthetics are okay to use.
Royal Purple is just taking a stab based on the info they have. Until Mazda verifies what they are saying, it is just educated guesswork.