Yes, someone cramming code into 4K ROMs in 1975 would very likely have been writing in assembly language. With you on that one.

Notably, Dave Nutting Associates (Bushnell's former employer, who also distributed Computer Space) had played around with an Intel 4004 in 1974 and then demonstrated (to Bally) a CPU based system with a frame buffer in September, which evolved into the Intel 8080-based board that ran Midway's Gun Fight. Atari would have probably been aware of this (Nutting Associates had filed a patent.) So, something along the lines of Intel 4004 or 8080 machine code, maybe M6800.

Anyone could in a Apple I from the monitor, even from the Kim-I and a serial adapter.