iOS file chooser for iCloud drive can't find tags
Finder tags are specific to OS X.

I'm almost certain the "tags" tab in the iCloud Drive picker does nothing right now. Tapping on it in Pages on iPhone 6 simply rearranged the folders, and I don't have any tags assigned to any files in Drive (either on OS X or elsewhere - all I have in Drive is the example iWork documents).

Even if it did do anything, I don't think it would do what you're thinking - that's a tab bar, meaning you select one of the choices. It's for sorting the presented list of files/folders, given that it contains Name and Date as well.

I also can't find anything on tags in relation to iCloud Drive, even for developers.