We actually don't know what knowledge people retained after the sundering. We do know that Amaurotians had various forms of record keeping that persisted even to the current era, so it's not like their accumulated knowledge suddenly disappeared overnight. It's just a question of how much of it would be practical and relevant for survival following the reduction in their magical abilities. You might have compiled the world's most advanced treatise on aquatic lifeforms, but what you're going to pass on to your children is the practical skillset required for survival, like how to cook food and find shelter using non-magical means. The more esoteric and less survival orientated parts of their knowledge would have faded from conscious memory over generations until it became actually relevant for society to learn them again.