I find their conclusion that taxes "weren't that much higher" not supported by their graph. In 2010 the rate they show was 30% (it is a bit higher now) and in the between 1945 and 1970 it was roughly 45%. Thats 50% higher. That's a lot higher.

given how "spiky" the graph is, your numbers are pretty much cherry-picking:

1945-1970 is ranging from 35% to 45% -- and it definitely won't average to 45%, which it only reaches once

2000-2017(?) is ranging from 30% to 42%

You've basically taken the high point of range 1 and compared it to the low point of range 2 -- and "a bit higher" doesn't really cut it when you're talking about what looks to be about 37% for the latest datapoint... about half the distance between your cherries.

And it's important to remember that changes in the effective rate aren't only due to changes in tax code. During a recession, when equities are doing poorly, you'll see a decrease in capital gains (which are taxed at a lower rate).