I'd mostly agree, but with them specifically calling out "not anything formal" as part of the expectations for interview attire wouldn't be the time I'd want to be riding the line of "is this too close to formal". This isn't a job at a tailor or stylist. You're not being tested on your understanding of the roles of various garments in different levels of fashion over time.
Presumably OP had seen/visited an Apple Store before and knew what employees wore there, so it's not a mystery what the uniform is, and therefore what is probably meant by "don't wear anything formal". It's not some kind of gotcha.
We might be getting a but pedantic about what “formal” meant at the time, but you would have had to be in that Apple culture circle to consider a button down formal. Seems normal today, but it was not back then in most parts of the world. Today I would agree that folks would already know the expectation.
Presumably OP had seen/visited an Apple Store before and knew what employees wore there, so it's not a mystery what the uniform is, and therefore what is probably meant by "don't wear anything formal". It's not some kind of gotcha.