In TL431 the cathode is not tied to the reference comparator input like in typical "reference diode" ICs. With a suitable divider, one can set it up for any voltage, like 100V. Or it can be used in a myriad other ways altogether. But it won't work very well if the cathode breakdown voltage is exceeded.
As for the power-on transient, I think it's only a matter of rise time of the power supply. If we pushed it down to nanoseconds, the output stage probably wouldn't turn on fast enough to protect the chip
More practically, I'm not sure where the safe limit lies but it's probably still much less than 1µs. Also, whether such brief avalanche causes any harm is another question.