Kids learn to read and write and do fractions by living. It's a part of life surely? You have to try really hard to NOT expose your children to these things. Unschooling parents (by and large) are not negligent - they read with their kids, play with them, bake and cook with them - most likely how most of our kids here picked up the reading, the ability to form letters and the concept of fractions, measurements etc.
No, they don't, apart from in a superficial way. A child may learn what half and 3/4 mean from baking, but he won't learn about the positions of fractions on a number line, how to add fractions with unlike denominators, and how they relate to each other. Nor will he learn how to think about complex mathematical ideas or how to write a persuasive essay. These skills come only with significant instruction and feedback from someone who's very good at them, and a lot of focused work on the student's part.
Claiming that you can teach these skills through baking or living is, IMO, creating a situation in which other people can be deprived of options because they haven't learned basic skills. Some unschooled kids seem to get lucky and get PhDs (in that industrial system criticized by unschoolers). Others end up shoveling horse manure because they still can't really read at age 19. Those two brothers were in the same family. If unschooling works so well, there shouldn't have been such huge gaps in outcomes.
Unschooling also strikes me as being similar to approaches to teaching girls circa 1700:
girls don't need a real education; that's for boys. So we teach them to read and write at home, and how to do the chores they'll need to do someday. That's a lot more outmoded than today's approaches in schools. Unless an unschooling parent is an expert in math, writing, literature, genetics, economics, and so on, it seems unlikely that the child will get a serious education. The superficial version seems like a more likely outcome, with some kids getting lucky.
