You are technically right in that Steam Input allows you to map every input for each game. But that's an additional layer Steam provides over the game. It's not the game natively supporting the controller itself like you get with the console maker's controllers. Ever notice how many games don't have proper icons for Steam Controller? Or how in game trackpad support is almost non existent? Steam does all the work to make the controller work and it's great, but it's not the same as the game natively supporting the controller.

yeah after getting the steam deck the steam controller is amazing! It could be better since the steam deck pad feels better, maybe this controller will be perfect. But no way I'm spending 100$ on a controller, the original steam deck 64 GB was like 300 something