TBF, it's not a specific bar. Any game can be better balanced, and, as I've said, perfection is not possible. I could never point to a game as an exemplar of balance, because it'll have things in it that are clearly imbalanced or could be better balanced.
It's easy to point out games that are badly balanced, and what's imbalanced about them, especially as the current edition of D&D is so well known and familiar and so imbalanced.
What was different about 1e AD&D is that there were so many attempts at imposing balancing factors. EGG
tried to balance the Paladin with RP restrictions & stat mins, tried to balance spell casting with casting time, interruption, all sorts of persnicketty little details like needing both hands or standing upright or not on a moving mount and IDK what all, there were so many and scattered about, tried to prop up fighter with favorable weighting of the random magic items table, etc, etc...
Subsequent editions have scraped most of that off, because it gets very inconvenient, was often circumvented anyway, and plain didn't work well. But, most subsequent edition haven't taken the obvious step of, sufficiently decreasing the power of the thing being arbitrarily/clumsily/ineffetually 'balanced' in some quixotic manner that seemed like a good idea in c1978, to a level that it would actually balance.
So, no, 1e wasn't balanced, but it
tried. It was a good faith attempt when the entire hobby had only existed for, like, half a decade.