Yes, in an open line you're the only one doing the talking
Radio communication isn't duplex, like a telephone call. It's exactly like using a walkie talkie. If you press the send button, only you can be heard, preventing all other conversation. If 2 people press the send button at the same time the messages get garbled.
At any given time, only 1 person can speak.
So you see, an 'open line' would make everybody privy to your panting and raving in a dogfight, but you wouldn't be able to hear a thing from anybody else.
How do you erroneously get an open line?
Switching the radioset to REMOTE and start sending by moving the switch on your microphone to the 'on' position would effectively give you an 'open line'. You'd have to move the switch back to the 'off' position once you've transmitted your message and 'close the line'.
As the switch on the microphone didn't automatically switch back to 'off' once released, the switch was sometimes forgotten.
In my opinion, that's what was meant in BoB and Piece of Cake.