Personally I think the 1 in 10 number is an over estimate. But I do think it's higher than people think.
It has surprised me just how many unexpected close matches mine and DHs family have come up with. So far. That's the other thing - you don't know what will pop up in the future either.
And there will be people with particular backgrounds and lifestyles who are far more likely to find out an unexpected secret. So rates will be higher in certain groups. If you marry someone who is older, already has a 2 year old and was in a biker gang, your odds of it 'all coming out the woodwork' are going to be much higher unfortunately. Back when the OP got together with her husband she may not have foreseen DNA tests, but she would have been well aware that he wasn't exactly good with condoms... I don't think after 20 years of knowing his history she can be too surprised.
It will affect her understanding of him though, because she's been made to confront a part of him which isn't nice. You can't dress that up. And the comments he is making NOW about those women aren't good. They ARE reflective of him and not the era.
She has a choice to make - decide to support him in blaming the women and villifying the motivations of these kids. Or to encourage him to reflect the current era and what we now expect of people. This doesn't mean trying to win Dad of the year but does mean acknowledging and making some sort of effort.
She needs to be aware that her children come from the current era and think in those terms.
It ultimately does reflect on her too for that reason.