How? Meteor's server-side architecture is still oriented around polling the DB, and I believe that's because many apps are still explicit request-response oriented.
You can find out more about the LiveUpdate core project of Meteor on their site [2] - it basically says the implementation of Live Updates for each db driver is independent to what the db is capable of. Specific mention of RethinkDB and Firebase is made as DBs that are built with making realtime data something that you get for relatively little work.
[1] https://github.com/meteor/meteor/blob/devel/packages/mongo/o...