Right, half is not "pretty much". Its meaning is more like "mostly", that's why it fits the above. It's something like, that situation is common enough it should be your base assumption when talking theoreticals.

For example, if you have 9 boys and 1 girl in a room, and pick one at random, you're not guaranteed to get a boy, but you're pretty much guaranteed to get a boy. It's a wishy-washy way of saying the most likely outcome by a decent margin.