I just tried this again today and found that it’s still not possible to bulk archive messages via mobile. I don’t know about others but to me it’s inconvenient - I get dozens of messages in my group inboxes and it becomes overwhelming if I don’t stay on top of it… which would be an easy task on mobile when on the go.
Maybe you are waiting to do it at some point in the future together with improving the bulk update options for messages generally? If that is going to take longer or is not a priority, then would it be possible to fix this now?