Hello forum,
To my knowledge the Pro Display XDR does not feature a hardware kalibration.
There are those that if can't directly 'turn the software knobs" of the hardware LUT means it doesn't have calibration.
In that context, yes. In the context can't do any calibration, not really.
"... Support for performing in-field recalibration of Pro Display XDR and creating custom reference modes, which includes adjusting color primaries, white point, luminance, and gamma, will be available in an upcoming version of macOS Catalina.. .."
Learn how to measure the calibration of your display. Depending on the needs of your workflow, you can also fine-tune the calibration or perform in-field recalibration on your display.
support.apple.com
That is likely a 'don't hold your breath waiting" version of macOS Catalina. There is probably also a delay and/or issues with specific hardware calibration equipment (drivers ) too over the short term.
Does that mean, the Pro Display XDR cannot be used as a "reference monitor" for
color grading jobs?
It can be for some jobs. In job for any context... not as much.