It is definitely the case. I guess that MFi does not prevent third-party chargers to work with a lightning cable, but it certainly does require authentication for data transfer.
Now, iOS supports ethernet-over-lightning, so you can plug an ethernet adapter and have it connect to your router. But it is illegal to make an accessory that would "integrate" an ethernet adapter. If you make an accessory, it has to go through the MFi protocol.
Here is an example: if you take a DJI drone and connect the remote controller to your iOS (lightning) device, it goes over MFi. It would be illegal (as per the Apple ToS) for DJI to use ethernet over lightning there.
Now, iOS supports ethernet-over-lightning, so you can plug an ethernet adapter and have it connect to your router. But it is illegal to make an accessory that would "integrate" an ethernet adapter. If you make an accessory, it has to go through the MFi protocol.
Here is an example: if you take a DJI drone and connect the remote controller to your iOS (lightning) device, it goes over MFi. It would be illegal (as per the Apple ToS) for DJI to use ethernet over lightning there.