We're talking with no adapters here, as in you have a single plain usb-c charger for all your phones.

Not if the cable is built into the power adapter. But if the power adapter just has a USB-C port, then you can charge it using the cable that comes in the iphone box.

Why feed the trolls? It's plainly obvious to everyone here that "charge X with Y" doesn't by default include special adapters or connectors.

Not really. The biggest claim is that this measure will reduce e-waste, but that’s a red-herring since the e-waste is in the charger not the cable, and chargers are already interchangeable.