"Unlikely" means either a) very small chance that the good thing will happen or (let's be charitable here) b) less than 50% chance it will happen.
So while yes, the chances of having a miscarriage are higher at 43 than at younger ages, and the chance of conceiving naturally are also low, (so yes, there's a less than 50% chance they'll have a biological child within, say, 3 years)
BUT the chances of having a child with a disability are still very low (definitely not 50/50) and there doesn't appear to be good evidence that older mothers have more children with ASD.
Mothers over 40 still only have a 1 in 100 chance of having a baby with Down Syndrome - adding that together with the chances of having a child with ASD that's not "unlikely".
And that's got nothing to do with your opinions on older fathers. A father can be good, bad, healthy, unhealthy, and suitable or unsuitable for a child - good dads don't all have healthy children and poor fathers don't have unhealthy children!
My DH as I've posted is an older dad and our DS has SEN. I'd say that my DH is the absolute best dad DS could have. Most of the younger dads I know don't have the life experience to support him either personally or in terms of knowledge of legal stuff etc. And many of the younger dads have a much shorter fuse. I'd say an ex-primary-school-head was a very good candidate for caring, supportive (and go-getting) dad.
So yes, their chances of conceiving and carrying a child are not as high as when younger. But their chances of having a child with a disability are still very low, and have nothing to do with how "good" the dad is.