I don't think they are talking about 5000 users chatting in one chat room, but about 5000+ users having a common place to talk to others in small groups and maybe read announcements in a main channel.
My university uses Slack, and the "main" rooms see a few posts a day, but there are tons of small groupchats and DMs.
My university uses Slack, and the "main" rooms see a few posts a day, but there are tons of small groupchats and DMs.