Seeing that 2 different factions may choose to form a temporary alliance in order to gang up on the 3rd faction, I find it reasonable to assume that we will be able to talk to eachother.

Reasonable indeed, but not necessarily true. What they meant by 2 factions being able to gang up on a 3rd faction is the simple dynamic of having 3 factions, and if one gets overpowered, the other two can "unofficially" stop attacking eachother and focus on the 3rd one. This was how it often worked in DAoC, but with NO means of communicating between factions whatsoever...
How did it work then? Simple, each server had a dedicated forum where people from top alliances in different factions could speak with one another. And so they talked to eachother, formed alliances, and coodinated attacks OUTSIDE the game entirely. It may be the case here, but it's a reasonable to assume that at least each faction will be able to "/talk" to eachother, and therefore communicate with enemy players that are "physically" standing nearby, though I find it highly unlikely that enemies will be able to message one another... Another way they communicated was that some people had multiple accounts, so they could log out of say their main "Midgard" account on that server, and log into another account that had a Hibernia character on that same server, and therefore act as an envoy for communication. That was the way they were able to communicate with people in-game...