For sure they will have used this (and just not have drawn it in the simplified diagram) as RS232 receivers would be very bad at pulling down the line, even to

For sure they will have used this (and just not have drawn it in the simplified diagram) as RS232 receivers would be very bad at pulling down the line, even to 0V. I cannot believe that this would have worked, even at slower baudrates, without any pulldown. But even with the pulldown, you will be limited in speed as you have to charge up the capacity of the "network cabling" to the idle level. Any resistor (high enough so that the RS232 drivers still can drive the cable to a sufficient "high" level) will need some time to reach the threshold of the RS232 receivers.