Yeah, you're bitbanging a 2-wire serial protocol that looks strikingly similar to I2C only without the part addressing...
Take a look at figure 21 - that shows how you tell it to read a register; you're manipulating SDCLK and SDIO while sending the address, then manipulating SDCLK and letting the part drive SDIO while reading the value out. Like I2C, the high bit of the address (in this case the register address, instead of the device address like the first byte of I2C transaction) tells the part whether it's a read or write operation. There's also an example of writing 1 page earlier (fig 19)