You are on target in my opinion. The resistance in the sensor circuit is negligible compared with the input impedance of the ECU. But the voltage is small and easily influenced by a ground loop caused by something sharing the same current path as the exhaust system to ECU, which includes the engine to body strap on older 200 series cars. Just a hundred millivolts difference would affect the switch point on the sensor's output. Later systems used a four-wire sensor providing its own ground return.

On the other hand, the resistance of the heater circuit is an appreciable part of how effective the heater is, so extending these wires with unreasonable lengths or inappropriate gauge and poor connections could keep the sensor out of its designed temperature range. You'd have to push your imagination to envision this being a problem with even the most adventurous do-it-yourselfers.
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Art Benstein near Baltimore Crrrrazy Ray's!