While the UNIX and Windows GUIs of the time were abstractions on top of command lines, thr Mac GUI of the time WAS the native UI. It was literally burned into the ROM of the machine. There was nothing lower they were hiding aside from the machine code.
There was a debugger BTW, the very limited programmer's key (which most users only used to kill the current app if it crashed). It could be replaced with the more powerful Macsbug.
Having the UI code in ROM doesn't preclude (and might actually benefit) shipping a simple but versatile sort of debugging tool with the OS. But Apple decided against doing so, making the system more appliance-like overall.
DOS's DEBUG isn't just a debugger; it also functions as a disk editor, hex editor, memory inspector, and general "system inspecting/tinkering" tool.
There was a debugger BTW, the very limited programmer's key (which most users only used to kill the current app if it crashed). It could be replaced with the more powerful Macsbug.