I know that. Not really up for a debate though.
We all know that Terrarian progresses from 5/0 to ~400/~150 and Steve only from 1/0 to 11(8+3)/80%, yet in game they function nearly the same. While the stats are obviously strong, due to the dragon/overlord being a strong eldritch, both might die multiple times in their epic fights when they're not careful enough.
Not sure where you got those numbers from.
Terrarian starts at 5 hearts of 20 hp each. 100 total hp.
Steve starts at 10 hearts of 2 hp each. 20 total hp.
You can convert one into the other by multiplying Steve's hp by 5.
In Minecraft's Normal difficulty, a standard zombie deals 3 damage, we can multiply this by 5 to get the equivalent of 15 damage for the Terrarian.
In Terraria's Classic difficulty, standard zombies deal anywhere from 10-18 damage, which is an average of 14.
As you can see, the difference is negligible, meaning they are easily equated. A terrarian's 5 heart start can be equated to Steve's 10 heart start. The main difference here is that the Terrarian is able to permanently increase their hit points beyond this total by a factor of 400% to reach 500 hp, while Steve cannot do this outside of usually prohibitively rare temporary buffs (the only one that he's able to craft are Golden Apples, which provide 2 hearts or the equivalent of 20 hp for the terrarian.)
We can use this to determine where the ender dragon would fall in terraria's progression:
Ender Dragon has 100 hearts or 200 hp. We can multiply this by 5 (which we proved is a valid method with the Zombie example) to get 10,000.
This puts it barely above Wall of Flesh's health pool(8,000). However, the hardmode boss with the lowest hp (Queen Slime) has a whopping 18,000 and is meant to be fought directly after Wall of Flesh, so we can infer that the Ender Dragon would probably be more comparable to Wall of Flesh rather than Queen Slime, and certainly nowhere near Moon Lord.
Let's look at the damage of both just to make sure. In Classic Mode, Wall of Flesh's contact damage is at 50, which is the equivalent of 10 damage for Minecraft or 5 hearts. Its projectile damage is at 22-30, which is the equivalent of 4.4-6 damage for minecraft or 2.2-3 hearts.
Now, to decide this, let's look at the Ender Dragon's damage values. We used the Classic mode values for Wall of Flesh, so we'll be looking at the Normal mode values for the Ender Dragon.
The Final Boss of Minecraft is equivalent in power to Wall of Flesh.
I rest my case.