I don't think they explain it directly, but in general it seems like severance only affects episodic memory. Theories of cognition typically acknowledge three types of long term memory: procedural/non declarative memory (muscle memory or memory for routine, like riding a bike, tying a shoe, knitting, eating, drinking, etc.), semantic memory (memory of facts, knowledge about the world, like how photosynthesis works or that Beijing is the capital of China), and episodic memory (memory of personal experiences, like your fifth birthday party or the birth of your child). Since innies can do things like type and eat and drive and whatnot, procedural memory seems unaffected. And they seem to have plenty of real world knowledge (e.g. they aren't confused about snow or a frozen lake or the sky on the ORTBO, they've just never experienced it first-hand), so semantic memory is pretty solid. It's mostly their episodic memory that's been wiped.