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
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/xgrin.gif.pagespeed.ic.QVVz6XIT20.png)
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.