I think if you read the statement carefully, it says the Mazda will get maximum performance from conventional oil. Then it references your local Mazda dealer. That doesn't preclude the fact that while syn oil doesn't improve performance it could result in longer engine and turbo life. If I recall early production cars with turbos, like Dodge, recommended full synthetic oil to prevent turbo bearing failure when the oil failed due to high heat, sometimes after engine shutdown.
They really just want people to visit their Mazda Service Provider and have them change their oil.