So, in the mode you describe, can the apple transmit back out serial?
It's a bootstrapping process. So as data is fed in via serial, the code running in RAM now will reply with certain characters to establish that the comms are OK. And in some cases, the bootstrapping process is entirely one way, where via serial the entire program blob is loaded into RAM.
Only once it's excuted does it begin to transmit back out the serial to the host.