He's not "in error" - he was British and that's what they used. And the book was in its 2nd edition - they sold over 10,000 copies of the first edition and I'm sure that "error" would have been pointed out by a lot of people if they thought he was wrong. BOAC bought 2,000 and gave then to every pilot and engineer.
Sure we use AoA these days but he knew what HE meant when he wrote "incidence"