Our household has a number of iPads and never had an issue with any non-Apple usb-c cable I’ve used to charge them with, mostly Anker branded but one or two AmazonBasics or Cable Matters brand. I’ve never seen an incompatible cable warning, my suspicion is it’s a cable that doesn’t have the right signaling to go above 5V, so it’s stuck charging at 5V and the iPad prefers to charge at 9v or 12v.