Rather than using his available Transistors to build simple AND, NOT and OR gates, what if he could rely on a single, superior gate? He had already learned about the NAND and NOR gates, which were 'universal gates'. They were way more versatile compared to dedicated AND, NOT and OR gates. With enough NAND gates alone, he could recreate the function of a NOT gate. With a different arrangement, he could also make an AND gate. The same was true for the NOR gates which could be used to build a NOT gate, and then an OR gate.
The difference might seem so small, and it might look like he was just adding unnecessary complexity to things, but on the grand scale of the fifteen million Transistors he had inbuilt, it would simplify his design massively, and also allow him to pack way more processing capability than anyone else could within the same space.
