Sure, the plaintiff may not agree with Valve's 30% cut because they find it too steep, but unlike the iOS/Android marketplaces, Valves is not a monopoly marketplace in the PC space. There are other competing marketplaces they can go to like Epic, Windows store, GoG (which most likely also charge 30%)
And plus, on PC you don't even need a marketplace to sell and run your game. Anyone can install software from whatever source. Heck, they can even host their games on google drive and share the link via e-mail to customers who buy their game off their website, or some ghetto solution like that if they wanna save a buck on distribution costs.
Their livelihood is not being gatekept or threatened by Valve or any other marketplace like in the mobile space. So I still don't get their monopoly angle here.