Thanks for bringing this up. We have a report: Use Groups in @mentions and Sharing which the dev team equates to Extend 'Mentions' to work with groups as well. Your input is welcome on these tickets and if you do vote or comment you will be notified when there is progress on them.
The workaround is to create users with the group distribution lists as email addresses. Then Confluence thinks you are sharing with a user and will resolve the name in the Share pulldown and associate it with the distribution list email. One comment expresses it this way:
We created user accounts mapped to the group email addresses so you can use them in mentions. The users need to have permissions to the spaces where they will be used either by adding them to specific groups (this can be tricky since you don't want to give them more permissions than the individuals in the group) or explicitly permissioning them to the spaces. To make it look more like a group in the UI we updated the avatar for the users to be a group icon.
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