There is no scope: operator in DEVONsphere Express as there are no scopes to work with.
You are looking at the correct documentation for DEVONsphere. However, if you have enabled searching DEVONthink databases, its search prefixes can be used, e.g., added:today. Those prefixes are in the Appendix > Search Operators and Search Prefixes section of DEVONthink’s help.
I will clarify that in the documentation for a future release.