Well, of course it is a guideline and not a straitjacket. The issue is that it guides DMs in the direction that they probably shouldn't allow player creativity, but if they allow it in the first place, they should limit or minimize its impact. The DMG offerst the guidance that creative ideas should get a (much) higher difficulty than standard solutions, and that they should only work for a single roll, and that they probably shouldn't count as a full success.
Yes, all of that is guidance. It also guides DMs into a direction I dislike. In my opinion, good DMs will ignore therse parts of the guidance. Essentially, the DMG guides towards points A, B, and C; and your posts indicate that you take the guidance towards A and ignore the guidance towards B and C. There's nothing wrong with that. It should not surprise you that other DMs may decide to take the guidance towards B and C, and ignore the guidance towards A.
Conversely, it would help if the DMG provides guidance on when players try to use a power or item in a skill challenge. But it does not; it is completely silent about them. So most DMs I've met are guided into either disallowing powers or items (essentially, you're now in a minigame where you can't do that), or they rule that you can spend your power or item, but you have to make a skill check anyway (and, as noted above, at a higher difficulty). So now the PCs spend limited resources to make things harder for themselves.
I note also that the latest official rule (RulCom/DMK) is that the plot proceeds the same way regardless of the outcome of the SC; the only consequence for failing is that the PCs lose healing surges or take a penalty the next combat. That was how WotC chose to "revamp" their adventures.
Again, this is guidance that I don't like; it guides in a direction that whatever the PCs do is not actually relevant to the outcome.
Interestingly, the 4E Organized Play team (who often appear more clueful about the rules than the devs) disagrees. Org Play gave the GM a lot of leeway in changing the adventure to make it more fitting to the PCs and/or more fun - and that includes changing the SC structure. Because also in Org Play, many GMs and players found SCs an unnecessary restriction.