it's not even SDI SDO, it's the fact that they didn't just say SPI, this is SPI, SPI is so generic and has so many ways of being setup that is is not even a standard protocol just a way of referring to a particular implementation of serial port that is generically called SPI, no one expects this to mean that given any two SPI ports they automatically talk to each other as there are 4 data sampling modes, 8 or 9 bit versions and the option to have the slave send data back. Which is no different to a UART/USART which in turn is not a strict definition but a collection of common features.