You have to write assembly code to get the most out of any computer, but getting the most out of a computer has become less necessary over time.

The Apple ][ was also open in the sense that it shipped with full schematics and an annotated listing of the ROM (https://archive.org/details/applerefjan78)

Inside Macintosh had a high-level description of the hardware, explained the memory map and how to call OS calls, and had good descriptions of the various data structures, but didn't go as far as including full schematics or a full listing. It also was a separate thing to buy, so most users wouldn't have it.