It's not clearly suicidal; they're helping to defend the town, not doing it single-handedly. And they do not have to confront the dragon.
How do they know that they are helping to defend a town? How do you know that they do not have to confront the dragon (without plot immunity)? If the dragon flies up to them and attacks, how do they avoid confronting the dragon?
In real life, a MiG is strafing a town. A platoon of men are setting fire to that town. Another platoon is attacking people.
There are (typically) 3 to 6 of you in a squad (i.e. a party). You have no air support. No backup. No heavy weapons. You know that your small arms are going to do practically nothing against a MiG.
You are going to take on a MiG and a few platoons of men fighting in the town on the off chance that there might be other soldiers in the town to assist?
Seriously? Without explicit orders to do so? My dad was in the army. He would laugh at this idea. Heroic tendencies, or no heroic tendencies. You come upon an obviously heavily superior force, you do not try to sneak up on it or go attack it unless you have orders to the contrary (or possibly info, like you know that another group is going to counterattack).
You are using metagaming knowledge about how the adventure is designed to make a decision when the PCs are far down the road and they cannot see what exactly is going on. They do know that a dragon is attacking. That's described right away (at least it was to us). And they are told as they get closer that creatures are moving through town attacking and setting fire to buildings. Our DM showed us a picture of a flying dragon and a smoking building.
That's what the players have to work with. They do not know that there will be other defenders, at least that description was not given to my group as we sat down the road looking at the town.
They do not know that they do not have to confront the dragon. That is metagaming knowledge that you just posted, but it is not part of what the PCs (or players) know.
From the PC's perspective, it is nearly 100% suicidal. Later on in the adventure, they find out that the dragon is really not interested in fighting, but that is not the initial set of information supplied by the adventure.
Do not use metagaming thinking. Use in character thinking and it becomes crystal clear that this is a suicide mission unless all of the PCs are super stealthy. Even then, dragons historically in D&D and hence in "in game" culture have very keen senses. Stealth should not even be an option (assuming the PCs make a nature check or some such to know that dragons have keen senses).