The point is that while of course they do make money from order flow, it's usually actually a small part of their revenue. Most of the revenue comes from interest on cash deposits (and other things), so even dropping trade commissions to zero won't materially reduce their profits.

Schwab's total 2018 revenue was over $10B; I expect they're more worried about losing potential and existing customers to the likes of RobinHood than they are about 1% of that revenue.