The 890 pro manual also says:

Note: Do not have more than one remote operating at the same time in the same room.

The manual doesn't cover that scenario but it implies that only the primary remote communicates with the RF extender and the secondary remotes send their commands through the primary remote. So in that case the master remote is keeping the smart state information - making it appear that the secondary remote "knows" that you're already in that activity when it's really the master remote that's keeping track of it.

So I think it does work as advertised but not because the remotes are communicating with each other and sharing state information. And I don't believe that will work with IR control (without the RF extender).