Objectively people with autism will have a harder life because of extra struggles. (Not that you can't have a good life, but you have a higher chance of struggling). The antinatalism movement is not about "improving the gene pool" or related to eugenicist ideals like you are implying, it is about reducing suffering. And the extreme conclusion is that the only certain way to reduce suffering is to stop breeding. (And not having children is not the same as some selective culling like you are implying is the 'spirit' of the post)
This isn't about exterminating autistic people, if the couple had adopted 3 autistic kids the op likely would not have had an issue. The op is pointing out how the desire to have "biological" children led to them doing a procedure that increased the likelyhood of having more than one child, and increased the likelyhood of complications.
Maybe "ruined 3 lives" is harsh, but I don't see this as eugenicist. It is standard antinatalism "having multiple children is bad when you could have adopted but your drive for 'blood children' led to this and they will now have a statistically more difficult life than their peers so you likely increased the net suffering in society out of selfishness"
Yes most autistic people shouldn't suffer if we dismantle the oppressive societal constructs and stigma around the condition and treat them as human.
Most people shouldn't suffer if we break down the social constructs and stigma around them, from race, gender and sex, class, and many other factors.
Antinatalism is not about selectively culling autistic people. It is about the realization that society sucks and the societal constructs we have are likely to increase suffering and so we shouldn't have kids anymore until those issues are resolved.
I am gay, and a racial minority, and an antinatalist. I would hate to have a child knowing they would likely have to face racial discrimination just as much as I do not find it moral to have a child because they may be gay, or autistic, or gender nonconforming, or poor. All those things would likely increase their suffering.
But I wouldn't mind adopting any of those kids, even an autistic child, because live people are people and deserve love and compassion. Antinatalism is about the non-alive children that don't exist and the stance that they shouldn’t ever come to be, no matter what they end up being because in our current world, live likely won't be easy, they would likely contribute to the global environment crisis, and will likely increase the suffering in the world. And also they cannot consent to being forced into existence.