The DEVONtechnologies folks will be curious to know the version number for macOS and DEVONthink, which you didn’t provide yet.
Looking for you in the “DEVONthink Manual”, release notes for Version 3.9.1 says:
As more than highlight annotations are summarized, the “Summarize Highlights” command menu has been renamed to Summarize Annotations in the Tools menu.
Does this help?