Such an iOS device could support USB-C charging, but e.g. only accept ethernet-over-usb from certified adapters.
I don't think that they can force the cable to be authentified, it's really just that there must be their authentication coprocessor on the accessory side.
Try to plug a USB-C keyboard into your laptop and use it as a webcam... because it connects with USB-C does not mean that it is a camera.
Nothing says that a smartphone has to support everything it can. Typically, ethernet-over-usb is not really a supported feature of Android. It can work, it works on some phones, and on others it seems like the kernel module is just not there (so it doesn't work at all). Is that an antitrust issue? I don't think so: the phone manufacturer just decided that their phones would not do ethernet-over-usb, that's their choice.
The amount of things that are optional on the protocol level is... basically everything.
If your USB-C plug only provides charging, it's USB-C compliant, and if I understand it correctly, will get a USB-C certification.