Duplicate files · Duplicates in iCloud Drive
Why are online-only iCloud files skipped when finding duplicates?
Short answer: Comparing a file means reading it, and reading an online-only iCloud file would make macOS download it. So Storage Studio only compares iCloud Drive files already on your Mac; online-only files stay online-only and aren't checked.
Comparing a file means reading its content, and an online-only iCloud file has no content on your Mac — reading it would make macOS download it, using space and bandwidth you were trying to save. So Storage Studio only compares iCloud Drive files that are already downloaded, and online-only files stay online-only.
That's why you may see "No downloaded duplicates in iCloud Drive" even if copies exist in iCloud. Duplicates that are only in the cloud don't take space on your Mac, though they do count against your iCloud storage plan.