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I'd expect the 0.1uF caps in the H72 sensor to be class 2 ceramic types as it's hard to imagine COG/NP0 being used here. This would presumably make the reactance at 10kHz susceptible to temperature changes and this might also worsen the drift vs temperature if you tried to use it with a 431 meter. So there's probably more than one reason the H72 can't be used with the 431 meter.

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I'm not sure how stable the ESR of the 0.1uF caps will be over temperature up at 50 MHz but presumably these caps were chosen carefully by HP. The ESR of these caps will contribute to the efficiency factor stamped on the sensor as the ESR in these caps will rob a tiny amount of the incident power to the thermistor. However, the ESR will probably be tiny at 50 MHz. The ESR will increase up towards 1 GHz and this will cause some loss in efficiency as the ESR of class 2 ceramic caps does tend to climb rapidly up in the GHz range.

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The standard sensor will use thermally stable caps here with low ESR and I would expect the loss resistance of these caps to be dominated by metal losses as the frequency is increased although the dielectric material will also contribute some loss. Normally the metal losses go up by sqrt(2) per octave and this is one way to try and model the efficiency vs frequency of the standard sensor. I plan to add this info at some point in this thread as the ESR does degrade at just worse than sqrt(2) per octave.