Then blacksmiths in the Middle Ages did not 'understand' the forging of swords. Only modern materials science allowed us to 'understand' why forging creates harder metal.
This is a semantic discussion about the meaning of 'understanding': does it mean you can globally explain how the system works and could come to understand the smallest detail of every part? Or does it mean you understand the smallest detail of every part?
The latter is a nonsensical definition: if that is the case, then nobody understands processors, because nobody understands transistors, because nobody understands quantum mechanics, because nobody understands why the fundamental forces act in certain ways. Nobody understands Newton's laws, nobody understands where babies come from and nobody understands what it means to perform a 'computation'[1].
Of course, that means the former was also a nonsensical defintion.