It seems the trick is to export from Photos as JPG on a local drive first, then open that image into Affinity Photo to edit!
I imported a photo from Mac Photos App (by dragging into Affinity Photo). Tone Mapping Persona requested an RGB Pixel Layer. With trial an error I created a Rasterised Image and selected that and could get into Tone Mapping Persona. The presets seem to show what I wanted, but clicking on any of them does not update the image! why? A bug? Screenshot included.