multi match means multi match. By multi they mean that if you have a lobby of 4 people, all 4 people are able to fight so nobody has to wait. If tag mode was added they would’ve said tag. Tag is definitely returning but for now they future update for lobbies is text chat and being able to have multiple matches at the same time.
I think they're gonna have to add special Arenas (well, it was the case even in the past) and/or apply some reductions to the models.
Previous DoA games since DoA2 had this streamline technology that allowed to load multiple models in the memory (only in certain stages that took less of that memory) to allow for Tag Team.
That technology was also used in Team Battle and, partially, in Survival Mode.
You can already see how Survival mode atm is pretty sloppy, which means they clarly decided to use a completely different technology/approach to allow (I assume?) for better, more detailed, more powerful models/effects in the non-tag modes.
Supposing my theory is correct, adding Tag Mode the way we know it, and as seamless as it has always been is certainly a possible feature (what is NOT possible these days, after all?) but it wouldn't be an easy task.
I wouldn't expect it anytime soon honestly.
As for the other features mentioned the multi-fight is exactely what you said.
The Text Chat is what it hints, the ability to type things into lobbies (atm there is no text chat in lobbies, which is preposterous, just voice chat)
These features are not yet available and will be released in the future.
(makes you wonder why they delayed the Lobbies so much though, especially considered through a glitch they were already "available" on PC, showing they didn't really need that much work after all, did they?)