Default MCabber setup can't complete names starting with @. That module adds to completion list all users with leading @ so they can be autocompleted and two special names: @channel and @everyone.

Package available in the AUR: https://aur4.archlinux.org/packages/mcabber-module-slack-completion.

Warning! Until Mcabber Issue 135 is not resolved, special build of MCabber is required.

Just add following lines to your MCabber config:

module load slack_completion