They really need to cite Mellon. He was an amazing economist. :(
Practically speaking, you're absolutely right, although he never, to the best of my knowledge, had any formal academic credentials in economics, nor published any scholarly works. His practical knowledge was acquired through his banking business activities(?).
Mellon should be considered the founder of supply-side economics in the US. He was advocating cutting marginal tax rates to boost national economic activity when Reagan was still a boy and Arthur Laffer hadn't yet been born (which is not to put down the latter two at all). It's just that Mellon receives absolutely no credit for his contributions, since the lefty media and lefty academia have been controlling the historical message, just as Thomas Sowell points out.
Aside from ideology, leftist historians of that era probably hate Mellon because Franklin D. Roosevelt's administration pursued him relentlessly for tax evasion, a case which fit the template of Roosevelt's class warfare meme very neatly. Mellon won a jury acquittal in his home town of Pittsburgh, but died shortly thereafter. Some say that his death was precipitated by the stress of the trial.