In pure white-room theorizing this is correct. However, if you don't apply the skill challenge to a game, what's the point?
Having a stable of interesting challenges? Thinking through the logic
of non-combat challenges to improve your improv skills? When you're tired or stressed or just completely forgot, you can pull out such a thing? When you're writing adventures for others to use, they can slot in smoothly whenever you're in need of a particular idea? Helping out someone else, who has gone looking for advice
? (Admittedly, that thread was long dead before I even saw it, let alone posted, but the principle is still valid.)
The difficulty is not fixed, though: the odds of each roll being a success or failure vary based on the character's level, due to its level advancing its skills.
Then there is literally no such thing as a "level-independent challenge" that can be described mechanically. Nothing like that exists. The only things you can possibly have are non-mechanical, and thus cannot be applied
, not any more than what I've said above; they are pure concepts, like "stop a ritual" or "negotiate with the ruler, which can only achieve application by giving them mechanics. By your own standard, the conversation is pointless.
I prefer to have a more productive conversation, where we can speak of things that lack explicit application to a specific game, but which can still be fleshed out to a significant degree without any reference to target numbers, modifiers, or whatever else.