If that were true, what would be the point of breeding? If every time I risked a female's life, she was going to lose 75% of her offspring, I wouldn't put her through that.
I believe a kit has a great chance of not surviving when first born than at weaning. During weaning they are old enough to eat pellets and drink water and not have do depend solely on mom's milk. As newborns they need mom's milk to survive
There is a small percentage of kits who die after weaning but it is more like 1-2% If the 75% was read somewhere it is really wrong and I would question the breeder who got those numbers severly. I think out of 76 kits born this year I have lost maybe 10 and only 1 of those was after weaning.