25% probability of producing homozygous zygotes is not the same as 25% chance of being retarded. You probably know that, which is why you left out the last word of the paragraph in your screenshot, just couldn't help but try to argue in bad faith.
> which can
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when these alleles are
deleterious.
This means that if you do not possess deleterious alleles, then you simply do not get congenital birth defects despite having recessive alleles outside of mutations (extremely simplified). There is also the case where even when inheriting said alleles the effects do not show and the offspring simply becomes a carrier.