All NBBO improvement requires is that if you send an order through the book you’ll get the best price on NBBO. Even when Robinhood got fined it wasn’t for that, it was for promising better than their competitors and then putting it in writing that they were happy being worse than their competitors.
Here is what robinhood advertises for their price improvement https://robinhood.com/us/en/about-us/our-execution-quality/
All NBBO improvement requires is that if you send an order through the book you’ll get the best price on NBBO. Even when Robinhood got fined it wasn’t for that, it was for promising better than their competitors and then putting it in writing that they were happy being worse than their competitors.