There seem to be no setting to between change default internal/external.
We dont "trust" the users to remember to add the :lock: emoji everytime there is an internal comment on a issue (work item..). It is also cumbersome, as most comments are internal discussions.
We have tried to change the external/internal property with an automation rule, but there is no way to identify the comments originating from agens in slack. the seem to be just from he agents user ids, and not from Atlassian Assist.