Illustrator 23.0.2. I use "cm" units in whole software, but Distribute Spacing right now working only in points...
I think your software is falling down year by year.
coming up on two years.. are they ever going to fix this? for stroke and type size and practically everything else you can type whatever units you want and it converts to document units, why is that not possible here?
The distribute spacing value still gets stuck in points. September 24, 2020. This bug was also documented at https://community.adobe.com/t5/illustrator/align-to-objects-unit-change/m-p/11053672?page=1.
As of April 21, 2020 this bug still persists over a year after the original report. Seems like it would be a simple fix...
This same issue has also been described at https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/37523554-distribute-spacing-in-align-tab-uses-wrong-units.