The author is wrong, that experiment doesn’t show anything about focusing. The blue channel in RGB is simply much less bright than the green, which means it has

The author is wrong, that experiment doesn’t show anything about focusing. The blue channel in RGB is simply much less bright than the green, which means it has much less contrast, which means that manipulating it in various ways has less of a noticeable effect on the image as a whole. This happens to be true for blurring it, but also adjusting the contrast, inverting it, pixelating it, offsetting it, averaging it completely, whatever manipulation you can think of.