Not 100% true.
It has an impact on lower and middle classes; it has basically no impact on the highest income brackets.
Compare a hedge fund manager who makes $1 Billion a year to 20,000 families who make $50,000 a year.
Is the hedge fund manager going to buy 20,000 homes? 20,000 people's worth of clothing and food? No.
You can change his tax rate from the current 17% (because it's capital gains, not income) to the 28% that the average families make and he's going to spend basically the exact same amount he was going to spend anyway.
However, if you dropped the tax rate on the 20,000 families making $50,000 a year from 28% to 17% - you WOULD see a stimulative effect on the economy.
This is why the Bush tax cuts did little to grow the economy. They were mis-targeted.
Seriously - if you walked up to me and gave me $20,000, I guarantee you it would be spent (most of it anyway). If you walked up to Nicole Kidman and gave her $20,000 you've done nothing to alter her lifestyle in the least. She might spend it, but it just means that another $20,000 will sit in her account unspent - thus creating no stimulative effect.