It make no sense because he used the character of a 2nd edition videogame to kill a 5e tabletop monster in his mind.
The power scaling on 5e makes no sense either. Peasants with magical weapons can kill Tiamat. I an not joking.
No they cannot.
Disclaimer: While making the maths I came across a massive point. Tiamat has frightening presence range 240 feet. With this every low level mob will shoot at her with disadvantage, taking the hitrate from a modest 5% to an abysmal 0.25%. I retract my point, it is literally impossible for any medieval warforce of medium size or lower to kill Tiamat. I will leave the math standing because math is fun.
Statistically they can pretty easily. 5E has no crit confirm, so all of them have a 5% chance to hit on a ranged attack. Now which damage dice they use is massive for how long they take, so I will do the math for 1d4 throwing weapons, 1d6 shortbows and 1d8 Longbows.
On a throwing weapon she takes 6 damage average from a crit, shortbow 8 and Longbow 10. So you need 103, 77 or 62 hits to kill her first turn before her modest regeneration kicks in. Multiplied by 20 for rough math you need somewhere from 2000 to 1200 peasants to take her out in a single turn. You do not need as many +1 weapons, since the peasants can pick them up from the dead and pass around after shooting. If you rule cheese with ready action a few hundred bows should be enough, realistically every second one should be armed.
Now Tiamat will likely be faster than many of them and get a heavy first hit. The question is how many casualties she can cause in a single turn. She will defenitly wants to use Divine Word on her action to kill everything in a 30ft radius around her. Also she has two 90ft cones of death in her legendary actions. With this killing her with anything that is not a longbow becomes unrealistic, since only those have the necessary 150ft small range to engage with her in a wide enough formation to not get annihilated. Otherwise she could kite the army with her 120ft flight to death.
So with some clever lineup you probably only need double the men to kill her in a turn. Since everything except +1 Longbows is out of the question you need about 2000 peasants to volley her out of the sky. Factoring in the lower population size, this is the population of a moderate city in the high middle ages. But if you march to kill a god that number can be gathered.
What frightening presence changes is that she uses her first turn to inflict fear on the entire army and completely annihilate their chances to hit her. So no, peasants with magical weapons can not kill Tiamat.