I am in my 60's, and this sort of comment really frustrates me:
"she is of the generation that would think he'll only know the pain of that if he experienced it himself."
There has been no generation like that in my living memory, and neither sets of my Grandparents ever thought like that either.
If my father was still alive - he would be nearly 100 years old - he would have found himself divorced pretty damn quickly if he had ever hit me, or any of my siblings. I can only remember my Dearest Mum losing her temper once with my bickeriing siblings and me (we had an open fire - don't worry, you can read on), she picked up the poker from the fireplace and thumped it down on a piece of wooden furniture near to us - I must have been about 8 years old at the time - I think that that shocked her as much as us, and her anger dissipated as quickly as it had come, but my siblings and I were equally as shocked, because that was the first (and last) time that she ever showed any sort of violent behaviour.
None of my friends' parents ever showed any violent tendencies. Since having my own children, I have never personally known any parents who have shown that sort of behaviour either.
Of course throughout my adult life I have seen children being smacked when we have been in supermarkets etc, but thank goodness not many, and those that were doing the smacking were often of a much younger generation than me, eg I saw it more between about the middle noughties to about 2018, than I ever did when my Dear Children were youngsters.
You say @UWhatNow that the MiL's behaviour was in a moment of emotional reaction, and that being bitten is very painful. Well yes, I know it is very painful, because when I was about 9 years old my Yorkshire Terrier bit me on the cheek whilst we were both on the sofa. My reaction was not to lash out, I sort of jumped up and exclaimed, and backed away a little, and by the time my DM came in from the kitchen having heard my exclamation - probably a loud "ow", or similar - I was telling my dog how sorry I was that I had either scared or annoyed him. The pain and shock from the bite had not made me react angrily at all, I genuinely felt sorry that I must have upset my poor dog.
Of course biting has to be discouraged, but maybe you and I have different interpretations of the word 'discipline', because in my mind you never discipline young children, and especially not those who are not NT. IMO it is every parents job to listen to, explain to, and gently try to teach their young children, how to behave appropriately in any given situation - or to just withdraw them from situations where explanations etc are not possible, or at least not at that moment in time.