Don`t worry. The explaination from said Bill Roberts is wrong (or so badly formulated, that I miss the message).
Put tell-tales on your sail, and you will see in which direction the air flows. I have never spotted a spanwise downward flow on a well trimmed pinhead sail.

Effect of taper ratio (and sweep) on wings is well examined, documented and understood in aeronautics since long (80 years or so). Google for "wing design" and/or "wing taper ratio".

Even without the theoretical background, naval architects found by evolution the positive effect of taper, e.g. in a gaff rig, which was for long the standart fore and aft rigg, several centuries ago.

And by the way, pinheads like aft swept masts. A well known fact for H16s and also discovered on many Bermuda rigged sloops (with pinheads) years ago.

If we would only listen more to our predecessors, technical advance would be twice as fast...

Cheers,

Klaus