they usually mean they don’t engage in anti-competitive practices.
But they do. They forbid devs to sell their games cheaper on other storefronts (outside of timed sales). Basically they enforce anti-competitive pricing on products in a way that makes it impossible for the devs to move the platform costs into consumer prices.
Devs could sell the product on Epic for example for $49 and make the same amount of profit as they do on Steam when priced $59 due to lower cut, but they can’t do it because Valve forbids it. It anti-competitively protects Valve and their 30% cut against competitors who would take lesser cuts, at the expense of end customers.