Nobody here knows it. Assuming, that the watch uses an optical sensor I can tell that the sensor's maximum sampling rate is 25Hz (Sense, I'm not sure if it is so for Ionic, I but as far as I remember it was no different). I don't know if the red-light (SpO2) sensor does anything for HRV measuring. Some research shows that the minimum sampling frequency for R-R intervals is 50Hz where the R-R intervals can be obtained with a reasonable error (for ECG equipment). But here's an interesting research result from this paper :
"The results could be interpreted that there were no differences in the frequency-domain analysis between reference HRV and PRV when PPG's sampling frequency was over 25 Hz. However, at fs<=20 Hz, significant differences were found for some of the variables"
Simply, 25Hz seems to be very minimum to obtain accuracy with the lowest possible (acceptable) error. Whether Ionic, Sense, or any other Fitbit device is accurate when capturing HRV, I can't tell but I really doubt it considering that a) it barely meets minimum requirements b) the sensor proves very often not being accurate even for HR. Sampling with a frequency of 25Hz leaves really no room for error.
Fitbit's HRV accuracy cannot be tested due to a lack of provided data and unknown methodology. As long as the data cannot be verified in the lab environment Fitbit's HRV is just a gimmick (but that's just my opinion).
