I worry you missed the "documenting tacit knowledge is extremely difficult" part. Tacit knowledge is notoriously hard to document in a way that successfully transfers the knowledge from one person to another. Pair programming is at least an order of magnitude more effective, if not more.

If two people build a feature, two people are slowly forgetting how and why until they move on, and nobody else gets it at all. It is easier than documentation (which I agree is hard enough to more than double the dev time), but it only postponed the problem a little without solving it.