As per the OP's quotes from the DMG, the DMG does say that the player needs to actually explain what is happening in the fiction.I can easily see how the impression got out there that skill challenges are scripted with little room for player creativity, especially if people just say, "I roll Diplomacy" (because it's on the list of approved skills), without bothering to explain what their character says and does and how that applies to the fiction in order to justify the mechanic of the die roll
And it's clear why this is so: if the player doesn't do that, the GM can't narrate a consequence that then establishes the circumstances for the next check.
I'm pretty sure they do, because I was doing this in early 2009 with nothing but the DMG to guide me, and I doubt that I made it up on my own! I haven't got my DMG ready-to-hand, and so can't remember if it's on p 42, in the skill challenge section, or somewhere else.including the DM considering the player's specific narration to apply modifiers to the skill check, which I don't think the rules even mention)
This issue of "unlocking", and the related issues of skills that can't work (eg Intimidate vs the duke) have been much discussed over the past 14 years. In technical design terms, to me it's like an invisible foe on a 4e battlefield - it's fair if the players have the chance to work out the parameters of the situation by deploying their resources; it's not fair if it's just a hosing by the GM. The borderline here is very context-specific.the very first example of a skill challenge describes how use of certain skills is needed to "unlock" use of other skills. That is perilously close to rigid scripting, and I think it was a huge mistake to set that precedent as one of mechanics over fiction.
In a physically-oriented challenge you might note that if the PCs grab the rope, then they can use it to climb down the cliff. The idea of unlocking skills, and unusable skills, in a socially-oriented skill challenge is the same sort of thing. It's not just skill 1 => skill 2; it's skill 1 => changes fiction => skill 2. A common complaint is "the fighter does push-ups to impress the duke"; the idea of "unlocking" is that someone first does something (eg Diplomacy-based) to make the fighter's prowess salient in the fictional context, and then the fighter can impress the duke with their push-ups or whatever. In an example I've linked to in this thread, the social characters engaging with the baron prompted him to declare himself a man of action, and the fighter PC was able to agree (and at that point I allowed the player to express that agreement via an Athletics check; I can't recall the precise fictional details of how that manifested).
I see the function of prep here being much the same as the "tactics" notes associated with monster stat blocks and published encounters in modules: 4e D&D is a fairly complex system and can benefit from the GM thinking in advance about how they are going to frame and narrate and resolve a situation that they are thinking of presenting.If anything, 4e is guilty of assuming a DM needs to prep skill challenges in detail ahead of time in order to run them effciently at the table.
So if you're envisaging the PCs might have to run to cover in a goblin-besieged homestead (as was the case for my second, perhaps third, session back in 2009), it makes sense to think what are some obvious skills the players might try and use? - eg Athletics, Acrobatics - and also what are some obvious things that might happen on a failure - eg encirclement by goblin wolf-riders. It's easy enough to incorporate unanticipated action declarations into a planned framework. Eg a PC uses a teleport ability (warlock or eladrin, say) and that grants a bonus to (or reduces the difficulty of) the Acro check.