Ahh I see. In the tutorial, you can see that the JPG file was saved with the "-2" in the filename as well. With the settings in the tutorial, the original unedited JPG would still be in the Contact Sheet, but it might just be off the side of the zoomed-in screen (I don't remember what the sort was for the Contact Sheet) As Hayo said, this is the only possibility for editing JPGs.
If I understand you correctly, then overwriting the existing file seems to be what you want. (Same as updating the original file, no?) As you pointed out, this is an option in LR.
-Mick