I have always assumed that the nautical preference for passing port to port, which also applies to powerboats, is the reason the port tack boat is burdened. Each boat would have to steer to starboard to cross port to port. That is normally easier for the port tack, as that boat comes off rather than having to come up, possibly forcing a tack. In Fig 33 maybe it would actually be easier for the boat on the really broad reach to give way as it can turn quite a bit before having to tack, while still passing port to port. But if both boats assessed the situation before getting as close as the diagram implies the boat on port tack could have easily come off and passed port to port. Maybe that interpretation is less complex to assess than figuring out if the stbd tack boat is "enough" off the wind, hence the current rule.