I believe Boyd is simply reiterating the classical position in the ring. I have always been taught (and always do in the ring), pinky toe touching the outside branch of the stirrup. My heels are always down, and leg steady. I normally ride in regular, heavy duty fillis irons.
When I ride in the wide base ones, I definitely feel like just my toes are in the iron in order to find that sweet spot where the heel becomes more flexible.
On the xc field, I always have my foot SLIGHTLY more “in” the stirrup than in the ring because at a gallop over terrain you need a little bit more security. I don’t know about you, but I’ve been riding in my 2 point with my foot in the proper closer to the toes position, and had my foot abruptly slide out of the iron (it was a muddy day that day). You do NOT want that to happen as you’re galloping between fences!
In short, I believe many eventers need help with their stadium jumping. Too many riders try to ride the stadium jumping like it’s the xc course; it requires a little more finesse and tact than that! I believe the “toes in the iron” is simply helping riders to gain a little more fine control over the leg.