I think their claims seem credible. I think Steam lowering their take shows that 30% was indeed higher than necessary. And lowering it for those selling shitloads of copies and keeping it high for smaller sellers does sound a bit backwards and scummy.
But both Epic and Valve are businesses. Of course they’re going to be greedy and scummy. I wouldn’t really expect anything else. I just think in this case the specific arguments towards Steam seem valid.