The fact that the best speed to fly interthermal is independent of the
headwind-tailwind component confused me at first too, and for the same
reason as one of the other posters. If your STF is 50 kts and you have
a 50 kt HW, you'll make no distance. The problem is, flying faster
than 50 kts (assuming it is the value predicted by MacCready's
optimization) will actually blow you backward. Why? You make headway
while flying, but when you stop to tank up before hitting the ground
(an assumption in the MacCready model that is not often explicitly
stated: you can find a thermal with the lift you've dialed in whenever
you want it -- not very realistic!), you are blown backward at 50 kts
due to the tilt of the thermal. Because you flew faster than optimal,
you must tank up longer and are blown back more.

When on final glide, the optimal speed to fly is different and clearly
depends on the wind. It no longer depends on the speed of the "next"
thermal, since there isn't one. It seems like a totally different
optimization to me.

Hope this helps.

Martin