Teaching Elder is likely right in the majors. What that third tweet says is:
"downhill plane", as measured by either release height or vertical release angle, has very little correlation to hitter contact rate for fastballs."
I suspect it's always been overvalued because it SEEMS to make sense -- and Cabbage, you did a good job of explaining the logic. But, I dunno, would Chris Sale or Randy Johnson be harder to hit if they threw over the top? I doubt it.