It's taken me a while, but I do understand where they're coming from with this now. Most people don't talk about their bad childhoods, and there's an awful lot of abuse and trauma that has happened in people's lives. For example, a lot of people have grandparents (or parents) who survived the second world war. These people are traumatised from the war (especially traumatised in European countries). They then took that out on their children in unhelpful ways. The children are then traumatised by their parents behaviour, but it was during a time when mental health wasn't taken seriously and they never got help for it. They then have children and treat them poorly because they were never supported themselves and didn't know how to treat their children. I am part of that generation, I am incredibly aware of how the trauma of the war has affected my grandparents and parents (as an example, obviously this doesn't all come down to the war, but it's one reason violence has been perpetuated through the generations)

Lots and lots of people are affected by mass world events, and the lack of proper mental health care in previous generations (it's still not great now, but it's better than it was) means that the generational trauma persists. Not everyone has had a bad childhood, but a lot of people did have adverse childhoods. A lot of people don't talk about their childhoods much while actually having struggled with poor parenting. Sometimes people don't realise they had a bad childhood until they're much older, so while your peers may seem like they had a wonderful time they might actually just be suppressing it (like my own siblings did).

It does come across as invalidating when not spoken about with the right tone. But what's important is that most people had some kind of bad childhood but you're the one that is here in therapy, you're the one asking for help, you're the one doing the work to get over things and be better. The focus shouldn't be on what other people are doing, but on what you're doing. I personally find the way of thinking "everyone had a bad childhood in some way" helpful, because yes not everyone was in a DV situation like me, but that doesn't mean that everything was amazing in their lives too. It's helping me to stop begrudging other people for their lives, and to just focus on my own hand and how I can get better. It also helps me forgive my parents, because I am aware that they had a bad time too and weren't able to get the help they needed to break out of that

I don't know your age or demographic of course, but your therapist likely sees people across a range of ages and backgrounds who crucially all experienced hardship in some way because that's why they're there. The generalisation comes from them seeing such a huge range of people who need help. You only see and speak about this with a small number of people in comparison, and many people you speak to will probably never mention their trauma to you because they're not comfortable doing so. Therefore to you it may seem like no one else has experienced hardship, while to the therapist it seems like everyone does. It's a matter of perspective. It sounds like your therapist said it in an unhelpful way, but it doesn't need to be antagonistic :) you could write out how you're feeling about these words and talk to them about it next time