That's kind of the idea -- if the (weak-ish) internal pull-up/-down can influence the ADC reading, then the pin's input impedance must be much higher than the internal resistor, which could mean "disconnected". It sets up a resistor divider where the low/high side is unknown but probably (over a short time) roughly constant. Such a probe would be pretty quick, and generally benign. It wouldn't tell you much if the sensor (ADC input) were expected to be very high impedance.
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