The Si4133 AUXOUT should not be treated as an SPI MISO line. The Si4133 has a 3-wire serial interface but it is not SPI. It does not support slave out data on the bus.
To read data from the part, you have to write special test instructions to the device, and then read them out AUXOUT. However, AUXOUT is not a true SPI MISO line.
Further, there are no test modes where AUXOUT can be set to tristate to a high impedance as would be required on a bus.
Please note that this article is applicable to all Si41xx RF Synthesizers including Si4112, Si4113, Si4122, and Si4123 in addition to the Si4133.