Actually, this redis runs in "master" mode on that port (from docker-compose). There are other redises launched on other parts that run in cluster mode.

Redis part of cluster:
https://github.com/wireapp/wire-server/blob/develop/deploy/dockerephemeral/docker-compose.yaml#L98
(ports 6373 - 6378)

Redis standalone in master mode on port 6379
https://github.com/wireapp/wire-server/blob/develop/deploy/dockerephemeral/docker-compose.yaml#L77