And the next couple sessions of the campaign involve just telling players they have to go on dangerous missions in the town as level 1 characters with no rest who have already weathered multiple encounters, when there's a castle full of other people who could instead, many of whom are trained guards who know the terrain. And then it railroads you into trying to fight the dragon when it's a dragon!
But I think the issue here is less that it removes player agency. The beginning of campaigns often have little in the way of agency, because they have to start somewhere. Had the scenario begun with the party staying in Greenest and it then being besieged by the cultists, trapping them, I'd consider that a strong opening and it has no more agency. The reason it's a terrible note to start the campaign on is that it requires a decision that goes against character for most characters (I think they also start as paid guards for a passing caravan, so their primary obligation is to protect the caravan not the town even if they somehow think they can take on an army and a dragon). It basically draws undue attention to the artifice of a campaign, by giving the players no motivation to do the thing it needs them to do, but then demanding they do it if they want to play the campaign. I'm fine with a campaign starting with a railroad, but it should be a railroad that makes character sense for most characters. The lack of agency in the opening scenes of a campaign isn't the problem. Forcing lots of inevitable arguments between the DM and anyone roleplaying worth a damn that end with "sorry, you have no agency and your character's personality isn't allowed to matter" is a huge problem.
At the beginning of the Rambo NES game it gives you the option to stay in prison rather than go on your mission. If you choose prison it just says "the game doesn't start until you say YES" to the mission. This is basically what our DM said when we played HotDQ.