That's what I'm saying. It is absolute. You're just using their level as an index. It's not causative.
I'm not sure if we're agreeing or not here.
If while designing your adventure you're starting with the mindset "the task/check should be 'difficult' for the characters that I know will be in this adventure" and then setting the DC to suit that without also justifying that difficulty in the fiction, then we're not in agreement.
But if you're of the mindset that "the fiction tells me what the difficulty of this task/check ought to be and so that's where I'll set it", completely agnostic of any characters that might ever have to do this task, then I have no disagreement.
So for example: if your adventure contains a slippery cliff that in the fiction would be pretty tough to climb for those without much skill, you might set the DC of climbing it at, say, 19* - whether the adventure was intended for 1st-3rd level characters or for 14th-16th.
* - yes, 19. There is no reason whatsoever to limit DCs only to numbers divisible by 5.
Yes. I literally said so. "You know that it is already supposed to be hard."
The table does not give you justifications. It is there only once you have justifications, and you need to know what number lines up with those justifications.
This isn't complicated. You only pick a DC when you know that the task is supposed to be difficult for its context!
I get this. And as long as that context doesn't include anything about the characters attempting the task, all is good.
This is like saying that giving monsters numbered CRs is causing them to be hard. No! Exactly the reverse! We know the monster must be hard, and so we figure out what numbers that is meant to represent, and then assign a number to communicate that difficulty. That's what the DC table is for. It does not tell you, "When the party is level 24, every single hard check should be X value." That's lunacy!
Conceptually, we agree on this much.
Instead, it tells you, "Oh, you've decided that you need a hard check, in the context that your level 24 character is facing? Alright. A level 24 character would find X value hard."
There's my issue. The fiction you're writing/designing should be what drives the DC, rather than the expected level of the characters that will be encountering it. So, if you're writing a high-level adventure* and you want to put some locks in that pose a real challenge for level-18 thief-types, then IMO you need to justify those stupendously difficult locks in the fiction in order to explain why lesser locksmiths didn't pick them ages ago.
* - and yes, this is a big reason why writing high-level adventures (or writing them well, anyway) is hard work: everything needs to be justified such that when players (in or out of character) ask about it, you've already provided the answer to the DM so she's not left floundering.
The table only comes into play when you already know what you need conceptually, you just need to give it a number so the mechanics can fire. It is literally exactly the same as a GM inventing numbers on the fly by having a really really good intuitive sense of how challenging the Lair of the Demon-Saint should be.
Yet again we're back to agreeing. If I'm designing the Demon-Saint's Lair (or inventing it on the fly, whichever) then yes, there's gonna be some damn tough challenges in there; and the fiction/lore will justify their presence even if I have to invent that on the fly as well. But if I'm running a published module then I-as-DM have a right, I think, to expect that fiction/lore to have been put in place for me: I shouldn't have to invent anything.
And note that the capabilities, levels, etc. of the characters aren't part of the equation; and if some low-level types have, despite warnings, blundered into this Lair then so be it: nice knowin' ya. Flip side: if the Lair proves to be a pushover for the PCs then so be that too.