Its possible that it would only react to him once he developed telepathy, which is exactly when he is no longer near it.Granted on both counts.
A question that comes to me is that if Ezren has established a connection to Earth Magic that lets him speak to animals... wouldn't the cube react to him like it did to Callum?
Whatever Ezren (and maybe his dad?) is doing must be something new.
I'm not entirely sure this works (and I would lose a lot of respect for the writers) but maybe it does react to him and Ezran and Callum are both aware of that but we have been kept in the dark. Its complete bullshit, but might not be impossible.
That said, yeah I think Ezran is using the dark arcanum to communicate with other living beings. Its probably something like "we are not alone and we will all live on in those around us". This is weaponized by dark magic to steal magical creatures' arcana but it can also be used to comfort and support each other.