The Mazda engine is a low compression diesel, 14:1. This greatly reduces combustion temps which greatly reduces NOX formation which is what SCR & DEF are responsible for getting rid of.
The Mazda Skyacti-D is supposed to have no SCR or DEF in the US market and the details are all over Mazda USA's website.
Lower compression will of course hurt mpg, but Mazda might totally make up for it thru great transmissions, a lower purchase price, a lower weight, and less maintenance related to the SCR system.