This is not true.
Here is a legitimate business selling schedule 1 drugs: https://www.caymanchem.com/product/10801/mephedrone-(hydroch...
Regulatory Information: DEA Schedule I
With a schedule I DEA license, you can buy this product, and the manufacturer can deposit your money into a bank.
That's why I'm not sure moving marijuana from schedule 1 to 2 or 3 will really change much from a banking perspective.
Marijuana dispensaries will still be violating federal law, no different than if they were selling sleeping pills illegally.