The way it reads and writes is by pulling the TX and RX lines low, without the resistor the MCU is "Floating" it's neither high or low. There can't be a pull up resistor inside the radio as the same connector on the radio is used for audio input and output. So the pull up resistor is put inside the data interface/cable.
If something is "Floating" it's very easy for a bit/byte to be interpreted incorrectly, there needs to be a large enough voltage swing to determine whether it is a 1 or 0.
If even one of those 1/0 get interpreted incorrectly the whole bit/byte is wrong and if that gets written to the radio then the programming is corrupt and will either stop the radio working completely or cause strange behaviour with the operation of the radio. In both cases it might be impossible to re-programme the radio again to get it working properly and the EEPROM or MCU may need to be removed from the board and reprogrammed on an external programmer or even replaced. The calibration data for the radio is also stored in this location.
The diode is used so the RX and TX lines do not influence each other when reading or writing. If they are just tied together, when you read from the radio the TX line on the USB to UART is getting pulled Low and when you write the RX line gets pulled Low with data. Some USB to UART chipsets don't like those lines tied together at all and will not work or crash. The CP2102 is one of those.
But you think well I can tie RX/TX together to do a loopback test, yes you can but you are not pulling them both High or Low at the same time on an external device.
Hopefully this helps and makes sense.
