Nice Strawman there. That isn't what I said. All it takes is one guy in a watch tower to ring the dragon bell while the dragon is still miles away. Hell, they don't even have to be warned. As long as they all eventually get to the dragon area and get their shots off, it still dies or runs away.
So they have a watchtower, with one guy keeping watch for dragons, and presumably other threats.
He sees a dragon: 3 miles away. Dragon’s speed is 80’, so at double move, it takes it 10 minutes to reach the centre of the village (As a DM, using round by round moves probably isn’t appropriate outside of combat, but whatever). The 90% of able-bodied population who is working the fields? Doesn’t have time to run home, get their longbows and assemble anywhere in 10 minutes.
Dragon is in town centre. Only 10% are close enough to home to be able to run home and grab a longbow. Of course, not all of them are brave enough to take potshots at a dragon, and of course, some may want to take extra time to ensure their families are safe, so, only 7% of the able-bodied population is armed and able to attack.
That population is spread out around the town. Partially, this reflects that people live all around the town. Partially, this reflects that bunching together is a good way to perish in flames. So at any given moment, 25% of the population doesn’t have line of sight to the dragon.
Dragon breathes fire a couple of times, sets the wooden town aflame, flies off. If particularly sadistic, takes a short rest, then flies back and kills the survivors will they are still putting out the flames on the smoldering wreckage of their town.