Actually, I'm looking at it with just 1 resistor & I don't see how it would work. If there's only a single resistor, wouldn't it pull both float switch inputs to ground even if only one of them triggered?
See how S1 is wired, float switch can be wired same way. This is enough for addressing floating ground. The other resistor provide safety from cases where the input (S1) has power (by accident). this is highly unlikey in your case i.e. you are unlikely to plugin something that has power (say a dc power supply ) in the ato sensor connector.
This is my primitive understanding , i'll let others chime in.