There appears to be an inconsistency about the description of SNIFF_ONT register in the 2.15 user manual.
In the 4.5.1 SNIFF mode chapter, there is written than 1 is automatically added to a SNIFF_ONT value. And your example there talks about writing 1 into SNIFF_ONT instead of 2.
But in the register map 7.2.30 Register file: 0x1D – SNIFF Mode there is nothing about it adding one and your example there for 50/50 duty cycle doesn’t use PAC value decreased by one.