How does "RS485 Pi" 2-wire transceiver connection work
This circuit indeed transmits a zero as a RS-485 zero signal and a one as an idle state.

Most RS-485 receivers have a (built-in or discrete) fail-safe circuit that reads the idle state as "1". So in most cases, this hat will appear to work.

However, without an actively-driven "1" signal, there is no noise margin (which would be the whole point of using a differential bus like RS-485).
So when you move your device from the lab bench to the real world, it's much more likely that the transmitted data will be corrupted by noise.



If you know the baud rate of the UART, it would be possible to construct a circuit that detects the start bit and then enables DE for the exact time needed to transmit the entire byte.
But in the general case, a RS-485 transceiver requires a separately conntrolled DE/R̅E̅ signal.