I have no doubt that that rank and file engineers were not aware of the underlying functionality that allowed for plain text content to be read.

Nobody would ever create a SendPlainTextToZuck() function that had to be called on every message.

It would be as simple as using a built in PRNG for client side key generation and then surreptitiously leaking the initial state (dozens of bytes) once in a nonce signing or something when authenticating with the server.