And any dungeon is exactly the same - if the only way for the players to earn XPs for their PCs is to assault the GM's dungeon, then they don't have any real choice. But in most D&D games this is not called railroading, it's called the start of the session.
And I think that Mearls' idea is this: in 3E and earlier editions, it is assumed that if the GM writes (or buys) the dungeon, the players will agree to have their PCs assault it, because if they don't, the game falls apart as there is no adventure for the session. So, with GM-driven Quests, it becomes possible to have the same basic play structure - that is, the players turn up and take their PCs through the GM's prepared adventure - for non-dungeon-bashes.
This is obviously not tactical railroading, nor even is it necessarily the sort of railroading that characterises poorly-written modules. At most, it is the same sort of rail-roading as one finds at the start of each of the G modules: "You have agreed to investigate the Giants, and here your party is at the entrance to their stronghold." Both the G modules and Mearls' Verbobonc Quest will be fun - and thus unlike railroading - provided that the GM, in choosing the adventure for that session, has a reasonable grasp of the sort of adventure his or her players enjoy (in the latter case, do they enjoy engaging in subterfuge for Cardinal Richelieu - sorry, the Archbishop of Verbobonc?).