Could you give a more detailed explanation about how the preamble detection threshold works?
After the receiver is enabled, the chip is looking for the preamble. If the chip receives the first preamble bits, then it settles the clock recovery circuit (adjust the data rate according to the transmit data, find the right edge changes to be able to get the bit sample from the received bits in correct time, etc.). It is called receiver settling time. This time is defined in byte period and the length of it is depends on the selected receiver mode: whether the AFC is enabled, whether antenna diversity is enabled, whether FSK or OOK modulation used, etc.
Once the receiver is settled correctly, the next step that the chip should be able to recognize that the preamble is received correctly. That's why the preamble detection threshold is used: if the chip receives at least the preamble threshold length of consecutive preamble bits without any bit error, then it considers that the preamble is detected (it can generate an interrupt for this event) and start looking for synchron word.
See the preamble detector section in the data sheet for more details.