I think if your ruler is major-spaced in px then the subdivision option does not do anything [probably because px is an absolute unit, there is no such defined unit hierarchy as in mm-cm--m]

if I set my major spacing to 1 px then what I se at 100 zoom is:

Click image for larger version.  Name: 2023-03-26 23_18_02-Xara Designer Pro X - [Untitled3 _].png  Views: 28  Size: 19.7 KB  ID: 133520