Did anyone even read the article? The point is that you cannot share someone else's charger anymore without risking an attack through the USB port. This was not an issue when the charge port was separate from the data ports. The article is correct in warning users from connecting this computer to untrusted chargers.
There is absolutely no risk in sharing someone else's Magsafe charger with the current Macs.
You don't understand. The primary concern is not that your charger may be "infected". The concern is that a borrowed charger may be modified to attack Macbooks that are being connected to it. That could be as simple as the charger acting as a USB keyboard and "typing" commands. Or it could act as a USB network adapter and highjack your Internet traffic. Or it could use USB-C's DMA to access the computer's memory. Or perhaps there is an exploit to attack the computer's USB controller itself. There are countless attack vectors.
It's been done (see e.g. here). Also note that since USB-C is not Apple-proprietary, it could be an entirely different charger as well. There will likely be many 3rd-party chargers for USB-C.
Probably not, since it's probably not a USB device out of the box. But if it is (e.g. acting as a USB hub), you don't know that it can't be modified. Almost all cheap USB devices today have writable firmware, and very few have protections against unauthorized modifications (e.g. by signing the firmware using cryptographic keys).
I don't think that's true. According to the people who demonstrated the "BadUSB" exploits, the vast majority of the USB controllers they looked at had writable firmware. Flashing the firmware during manufacturing may actually be cheaper than producing a PROM.
The problem with USB-C chargers is that the user has no way of telling whether it's just a charger (short of taking it apart). This problem only arises since the new Macbook doesn't have a separate charge port anymore.
