That's because gold bounties and long respawn timers equalize games if you manage to kill the opposing team's big heroes. Once you get to the late game enemy fortifications melt under any real pressure so it's much more about forcing buy backs so they'll be out of position or force when you strike.
I don't think of a game as complex as Dota 2 as being balanced though: the meta constantly evolved and has to be tweaked. Even with very simple board games with unchanging rules the metagame evolves, so I don't think there's any objective frame of reference for what perfect balance would be.
That said, Dota benefits from more than a decade of balance adjustments, so it rarely swings heavily especially in regards to pro play at this point, compared to less established metagames.
The bigger thing that makes dota better at this than league is the way itemization works in the game.
In league items get more efficient as they get more expensive in terms of stat per gold.
In Dota, items get less efficient. The most efficient stat per gold item in the game is the ironwood branch. in Dota you spend more to have your items take up fewer item slots, not to get more stats. Also stats in general in dota are of less value because of the power of the abilities. One of the best items in the game gives no stats, and many more of the items are desirable more for their active abilities than their stats.
This seems like a clear design mistake to me, and it makes league games largely either completely one-sided stomps or super conservative low risk games decided by a single teamfight after the teams get so strong they can kill an entire lane and the whole base in a single death timer(which is another issue dota has addressed, with buybacks, which admittedly seems kludgey, but if it makes the games better, who cares?).
As for the OP's question, it always depends on what you mean by balanced. If you consider ideal balance to be the largest possible number of viable combinations, I think Dota is pretty damned balanced. I certainly have never played a game that had as many options that all felt viable where it still felt that my decisions mattered.
Obviously a 100% symmetrical game(in real time, turn based games have innate imbalance even if symmetrical) is more balanced than an asymmetrical game, but it also has fewer strategic decisions to be make. Dota has what some would argue is a complete overabundance of choices, and yet almost anything can work.
What are your quantifiable metrics for that? Prize pools aren't everything.
Dota just had open qualifiers for the international, with a thousand or so teams from each region trying to get an invite. In league if you aren't an LCS team or LCK or whatever, good luck ever becoming one. In Dota anyone can form a team and compete on an even playing field with the best teams in the world.
The dota 2 competitive scene is larger and more healthy just because it is so much more inclusive.
_/¯
