No they weren't. If we look at it from the perspective of pipelines (and not interactive programs that take input directly from the user and display output on the screen), stdin is for receiving data from the program in the pipeline before you, and stdout is for sending data to the thing in the pipeline after you. That's not bidirectional, that's a unidirectional flow.
There is no design/hardware/software limitation to reading and writing to them at the same time. That's your bidirectional channel with that one process.
>stdin is for receiving data from the program in the pipeline before you, and stdout is for sending data to the thing in the pipeline after you
No they weren't. If we look at it from the perspective of pipelines (and not interactive programs that take input directly from the user and display output on the screen), stdin is for receiving data from the program in the pipeline before you, and stdout is for sending data to the thing in the pipeline after you. That's not bidirectional, that's a unidirectional flow.