My understanding is that the calibration could only be feasibly carried out for smaller circuits where simulating the circuit classically was possible. At issue is whether the calibration is generalizable; whether the calibration carried out for a particular small circuit C was reused to simulate another circuit C', or whether fresh calibrations were carried out.
Page 17 of the supplemental data [1] shows the calibration process in schematic, and appears to indicate that the calibration was necessary for each input circuit C, since the graph showing the fidelity improving with iterations of the calibration is labelled "b, Data from a two-qubit XEB experiment." The water is admittedly muddy, and hopefully clarification will be forthcoming from some quarter to determine whether Kalai's particular critique has substance.
Page 17 of the supplemental data [1] shows the calibration process in schematic, and appears to indicate that the calibration was necessary for each input circuit C, since the graph showing the fidelity improving with iterations of the calibration is labelled "b, Data from a two-qubit XEB experiment." The water is admittedly muddy, and hopefully clarification will be forthcoming from some quarter to determine whether Kalai's particular critique has substance.
[1] https://static-content.springer.com/esm/art%3A10.1038%2Fs415...