Having just read the book, this description is erroneous. Morley does propose the first variant givn on this page, but it is an alternative proposal, offered almost reluctantly after a digression on Knight's Tours, to the one the book is ‘about’ (though in many ways it's really a long and winding — though charming — book of Chess‐variant apologetics); the main variant in the book has only the ‘corridors’ on the sides, not the ones behind the camps. The second version offered here would seem to be apocryphal.

The corridors originate, according to Morley, as a way of giving the Rooks' pawns the ability of capturing in both directions, as the other 6 pawns can. The promotion rule is, as suggested here, that pawns can promote on the enemy back rank — pawns are given the opportunity to capture into the corridor ‘at their own risk’.

The variant as a whole is intended to expand the possibilities of the game (and counteract its being ‘played out’ — not so much between players of equal strength, as between players of potentially equal intuitive ability but differing levels of learnedness, esp. re the Opening) without making any changes to the rules or pieces, in contrast to e.g. Henry Bird's earlier proposal (referred to in the text) which introduces the Carrera compounds, as the added complexity would in his opinion be, though perhaps interesting to experts, too intimidating for lay chessplayers.

As a change, the ‘inverse Gustavian’ board (i.e. adding everything except the corners on each side) is a nice way of accomplishing that goal imo (and indeed, the Gustavian board has the opposite goal: introducing new pieces while changing the board as little as possible).