Well, yes indeed it is. The conclusion of that should not be to tax capital gains more, it should be to tax income less. If you tax capital gains more, people will invest less in things that produce capital gains, which means less economic activity, fewer jobs, lower wages, etc.
It has been soundly proved wrong. We have been coddling them by cutting cap gain taxes for 30 years. World is awash with capital. 2 trillion dollars uninvested. Labor is cheap, capital is cheap now. There is no demand for goods and services, so there are no good investment opportunities available. Why? Because the super rich have vacuumed up all the gains of all the productivity gains leaving the consumers threadbare.
Must tax the rich and spend it, spend it wantonly, spending it building bridges to nowhere. The Feds can bury boxes full of cash at random locations and ask people to go dig and find it. Even that would produce more economic growth than this madness of begging the rich to invest and create jobs.