Where that line is, between Hard Authority and Soft Authority, and how much is too much varies from group to group, and player to player, and sometimes even session to session.
But yes, sometimes there's only apparent consensus because one individual's personality overwhelms everyone else. If they want to run the game they should step up and DM.
I'm no expert on group dynamics. But I think it's not at all uncommon in many different kinds of social situations for a natural "leader" role to emerge, and very often that falls to whoever has the strongest personality/charisma/willpower or the most expertise concerning whatever the group's "project" is. In D&D groups, the DM is all too often both of these.
Speaking personally, I typically know far more about the rules than any single player I referee for. (Not that there's a whole lot there to memorize if I'm running OD&D, but at least I do my homework. Whereas playing a character in that system hardly requires that level of effort.) Typically, I find that this is enough to maintain an unlimited supply of (what
@Michael Silverbane calls) both "Soft" and "Hard" Authority over the campaign.
The exercise of Hard Authority on my part might run into a limit if I were to encounter a player who was adversarial enough in their approach to my campaign that they felt the need to get into a "contest of wills" concerning a ruling I've made or a restriction I have in place. But it's never happened. Likely for the same reason most players don't bother to read the rules: they're just there to play and they tend to go along with whatever to move the game along.
So, having never been in the unfortunate position of getting into a childish pissing-contest over an aspect of an elf-game, I can't say whether or not I'd automatically win by virtue of being the DM. And if I
did get into such a contest of wills and then win, I couldn't say whether it was
because I was the DM and thus "the" authority over that campaign; because I've been a DM for a long time and am therefore in some sense "an" authority over reffing campaigns; or pure, willful cussedness on my part outlasting the hypothetical adversary. There would be no way to tell for certain.
Does anyone hold the position that the DM is the ultimate authority? It seems to me that people are saying more that the DM is the final authority, and other people saying “I can’t believe people run their games so dictatorially!”
So what's the difference between "Ultimate Authority" and "Referee that creates the campaign world people will be playing in"? Is there one?
"Ultimate" and "final" are synonyms in this case. That's what "ultimate" means:
final. That's why second-from-final is called "penultimate" and third-from-final is called "prepenultimate" etc.
Now as for
absolute authority, that's a different matter. Yes, I think there can be a difference between an Absolute Authority and a referee who worldbuilds the campaign milieu. The mere fact of worldbuilding the setting doesn't actually imply anything about how heavy-handed or collaborative the referee will be when it comes to either selecting/designing/house-ruling game mechanics or applying and adjudicating them in-game.
For a DM to be an Absolute Authority over the campaign, they'd have to approach it in a certain very traditional fashion, being both the worldbuilder of the setting and the "keeper" of the rules (in the old sense that the DMG and MM were "for DM's eyes only" and players ought not to peek at either), soliciting no player input regarding either setting lore or game rules.
This actually raises a rather interesting question for the more modern and collaborative clique of DMs: do you have every expectation that the players will be reading through the DMG and MM, looking for magic items that they want (maybe even requesting and then expecting to "find" them), learning and memorizing monster stats?