Interesting read, good to see the focus on user research.
If the intent was to reduce the icon footprint, I'm curious what research they had around:
a) using the chevrons instead of arrows for pagination
b) why the red x wasn't a circle with a slash through it (to avoid confusion with the close icon)
c) why some icons have dark fill (e.g. plus and minus) rather than hollow fill
I'm also curious how the arrow right circle, how well that tested positioned left vs. right of the CTA text (unless it is just a style thing to keep it on the left).
Ben Cullimore, if you happen to read this, would love to get your advice on this!