Thanks for that postscript. Yes, some OA book publishers request or require subventions (aka book processing charges or BPCs) and some don't.

Indeed! Yeah there was initially a link to Lever Press in there as an example of a diamond/platinum OA press, discussion of how one may not need to pay for green OA—but alas. And then some presses also don’t call OA funding a subvention at all, which makes sense. So much to cover!