Not entirely true. Batteries are still made up of rare and polluting materials. And mining is not exactly ethical either
Edit: I feel my comment doesn't give enough credit to the one above. We really should have replaceable batteries and devices that can be repaired easily. It's just not a reason to not take care of the current battery if we can and to a certain extend
If the steam deck charger has a voltage output option that matches your phone (5V) then there is no issue. Current is set by the load in this case since the power convertor in the charger is a tension (voltage) generator, not a current generator. So the phone won't get too much current