If you're dealing with constant velocities, you are in the territory of special relativity. In this world there are no event horizons and every object can interact with every other. If two galaxies are each receding in opposite directions from a third central one at 2/3 c they will each see the other receding at 12/13 c according to https://en.wikipedia.org/wiki/... (section 2). Velocities do not add up the way you think they do and when they get to a decent proportion of light speed it starts to matter. This has been experimentally checked using moving atomic clocks. Thus they can keep on exchanging messages, although the messages will be quite redshifted when they arrive and take longer and longer to make the journey.

However, the original article deals with accelerating motions, since that is what the universe seems to be doing. This is crucial.

One way of seeing what happens is to imagine two galaxies accelerating away from one another. Assume there are clocks freely falling in both galaxies.

Define a function f so that a signal sent from one galaxy at lightspeed (could be photons, gravitons, neutrinos, doesn't matter) at time t on the local clock arrives at the other galaxy at time f(t) on its local clock. It's not hard (for anyone with a degree in astrophysics) to work out exactly what function f is. It turns out that there is critical time T such that as t approaches t from below, f(t) approaches positive infinity. In other words the last few signals emitted by one galaxy as it's clock ticks towards T are spread out across the whole of the rest of time when they finally catch the other galaxy and no signal emitted at or after time T can ever arrive. The critical time T depends on the current separation, velocity and acceleration of the galaxies in a fairly straightforward way. After local time T nothing you do can affect the other galaxy. After its time T you can never find out what happened to it.