That's why I said both.I think it is neither.
I think the designer intent is absolutely to deliver a reasonable, balanced, equitable game where everyone contributes meaningfully and no one has an inherent leg up nor needs special DM intervention.
However, I also think that this result is not some totally unforeseeable unfortunate accident that could never have been avoided or dealt with.
Instead, as I've said, I think it's the combination of several factors:
1. Intentionally doing things that sound positive and beneficial in isolation but which link together to produce problems.
2. Preserving, reviving, or enhancing "tradition" without actually examining whether that tradition is beneficial, or whether it has aspects that could be mitigated.
3. Subconscious motives that encourage greater power for spellcasting and no gain (or even sometimes loss) of power for non-spellcasting.
4. Biased thinking, particularly biases built on thinking everyone will play in certain ways or deal with problems in the same fashion.
5. Faulty understanding/application of statistics and mathematics more generally.
6. Failure to account for player psychology in the design.
None of these require "let's make an unbalanced game!" as an intentional motive, and yet all of them are the product of intentional (if sometimes heedless) actions. Which is why I say it is intentional, but not because the intent was to make an inequitable game.
The intention was a little purposeful imbalance what would be shrouded by playing the way the designers play.
The mistake was them making too many assumptions based on themselves and accidentally creating imbalance that is barely contained by the limits of the guidelines.