Yeah, because expecting heroes to act heroic is such a huge stretch.
The point is different from this.
Expecting all PC heroes to act suicidal is a huge stretch.
I know that at our table, a few of us (about 2 out of 6) tend to get into character and stay there. Something that is opposed to character concept jars at some of us.
I was playing a LG wizard. My PC has an Int of 16. I'm all for helping out the greater good, but Int 16 going into a burning town with raiders and a dragon is just plain suicidal for no observable gain. Just 6 more dead creatures. Maybe we can take out a few raiders, but the dragon just might smoke us before we even set foot in the town. As a player, I know that it is ok, otherwise the DM would not run it. But as a PC, my character does not know this.
I prefer playing the character via character knowledge, not player knowledge. I suspect that the "this is suicide" thought never even entered the minds of many players at many tables. It entered our minds and our PC conversation.
The whole "but you are heroes" argument might not fly. Let's take 911 as an example. Informed decisions are different than uninformed ones. If a structural engineer would have gone to the command post in NYC that day and said "That building is coming down in less than 10 minutes. You've got to get your men out now and get everyone away from these buildings now or hundreds will die.", it's likely that whomever was in charge would have made the right call and saved a lot of rescuers lives. Taking on a dragon is that level of suicidal for first level PCs.
Taking away the metagaming knowledge that "it's ok, it's a module" puts the game into one of suicide for the PCs. If the dragon would have attacked for real, most PCs at most tables would have been killed.
That is what the PCs should expect and that's why the module is subpar. It requires metagaming knowledge to even get the adventure started. At least at some tables.