I have one sat here. The earth terminates at the disk and does nothing!
As you can see from the pictures. The disk does not pass through the plastic so the adaptor is NOT earthed in any way!
Images from indestructibles.com
I believe that it is my duty to Educate you on Apple Magsafe Power Adaptors, and to put a stop to this before you confuse any other users.
First.
You forgot to add 1 important picture from instructables. If you zoom in, you can see a black cable coming out of that white "plastic" you talked about. You know, angle is everything! (Every time I refer to "plastic", I am referring to the plastic that you claim covers the metal disc completely not allowing for a ground connection).
Second.
Here are some beautiful pictures of the black cable that comes out from the "plastic" which is connected to the AC adapter through the metal disk. If a DC cable (provided by Apple) is used, then you adapter is grounded! Wow! Note that the last picture is the best.
Not yet convinced? Don't worry, I got you covered. I got the screenshots between 4 and 5 minutes.
Try watching the YouTube video!! Here.
Third.
Logic, always put logic before all things.
Why would Apple waste money on adding a 3rd pin on their cable extension cord? Because if your claim were to be true, then it would be pointless.
Forth.
The magsafe pin, either 1 or 2, or T or L, however you wish to call it, has 2 ground pins, the outermost pins.
Screenshot taken fro Wikipedia without permission from the owner. Muahahah!
Wikipedia. But I am quoting it, though!
I hope that this has helped you extend your knowledge on Apple Adapters; for next time, please do more research before grabbing the first pictures that you find on the web.
Questions welcomed.