I’m not sure that kind of thinking is really particularly helpful to your DS.
You need to take practical steps to make sure his boundaries are respected:
Go and see the teacher about the problem and get her to deal with personal space at their carpet time (or equivalent)
Go back to the mother (calmly) and explain the problem. Apologise for the kick and ask her to instruct her daughter to stop hugging him as it is distressing to him. Tell her you’ve raised the issue with school.
Role play with your DS how to avoid hugs (step back, hand out, firm “no, hugs”)
Work with your DS on his anger management. Kicking may be understandable in this situation but it’s not acceptable. He’s 8yo, old enough to know better and put strategies in place.
To return to your final point. Parenting isn’t just about teaching your children what is acceptable now, it’s about building foundation blocks for future behaviour.
I have a twin son and daughter. We have always been equally strict with them about the no hitting/no kicking/no pushing rules.
We have always been equally strict that “no” and “stop” must be instantly respected.
However we have also had additional conversations with them both about the difference between the sexes both in terms of physical strength and in terms of cultural reactions.
If my DS hit my DD at 5 yo he wouldn’t have hurt her any more than she could hurt him. However we explained to him that as he grew he would become considerably taller and stronger than his sister and that there would be no equality in any physical fight. As a result it was less acceptable for him to hit his sister or any girl because one day soon he could really, really hurt them. He’s 11yo now, he’d never lay a finger on her, he’s 6 inches taller and considerably stronger.
Similarly we told our DD that most boys are raised not to hit girls, told that it’s a shameful thing to do. And that most boys will not hit a girl, because she is a girl. We explained to her that hitting a boy who won’t/can’t hit you back (other than in extreme circumstances) is unacceptable.
Both children have been told that physical violence is forbidden other than if you can’t defend yourself in any other way (ie you are pinned, can’t run away and they are already hurting you)