Fascinating! You say in your review that they don't mention green manures as such, but they do seem to be deliberately permitting weeds for a similar purpose.
Yes, that's right. It's odd that such an experienced gardener doesn't mention specific green manures but I suppose he would ensure the weeds went to compost before seeding. Mind you, by 1943 staffing would have been an issue.
They were lucky they could retain the topiary rather than having to grub it up for veggies like so many parks and gardens.