The reason for programs like dvdshrink is, that most movies are mastered on 9,4gb dvd-r discs. Consumer level dvd-r are 4,7gb. So, if the movie (with all the extra stuff, or even without) is 6gb, you need to re-encode it to fit on a 4,7gb media.
I haven't read the articles, so I'll guess

The first article discusses dvd's that are singlelayer (meaning, they will fit without re-encode to a 4,7gb disc). And the second article discusses how to re-endoce duallayer discs that are more than 4,7gb in size.