I think the "missing" information from the skill DC guidance is a discussion of what level is appropriate. Obviously, party level is the right level for "appropriate" challenges but good adventure design also includes the options of going after challenges that are easier or more difficult than appropriate.
For example, if getting information about a prisoner in the local lord's gaol is of medium difficulty at level 5, it doesn't become harder to pump those guards for information just because the party reached level 8. Instead, they should get the benefit of their improved capabilities.
Similarly, you could have a skill challenge (say - getting the prisoner freed by arguing successfully at trial) that might be beyond a typical party of the PC's level. For example, that might be a level 10 or 11 skill challenge for the level 8 party (maybe because the GM thinks breaking the prisoner out is more fun). However, if a party is particularly diplomacy/bluff focused, they could have a decent chance of succeeding.
This was a bit irrelevant on the old charts, since pushing the DCs from medium to hard was the equivalent of making the DCs a few levels higher. Because the new charts don't increase at the same rate, it becomes relevant to know when to move up/down in level instead of left/right in difficulty.
-KS