Well, if you actually read the document, that's not how NaN is encoded in these data types--these are not IEEE 754-compliant encodings.
As for why not posits, I'm not entirely sure, but the "variably-encoded exponent width" nature of posits likely makes several details of their construction in hardware more difficult than things that have fixed exponent and mantissa widths. Although by the time you're talking about 8-bit datatypes, implementing operations by lookup table starts to look appealing.
Ahh okay wasn't aware the encodings were different, after taking a glance at spec it seems much better than what I pessimistically envisioned.
2 NaNs, and -0 still seem like a bad call (-0 could have been the singular NaN), but I guess I understand maybe why they don't want to deviate too much from IEEE754 floats.
As for why not posits, I'm not entirely sure, but the "variably-encoded exponent width" nature of posits likely makes several details of their construction in hardware more difficult than things that have fixed exponent and mantissa widths. Although by the time you're talking about 8-bit datatypes, implementing operations by lookup table starts to look appealing.