I don't think the tree is absorbing the post. I suspect there was a second stem to the tree, which either died or was cut by a previous owner (for whatever reason). The remaining stem has been growing around the cut stem. It has well defined callous wood grounding around the edge of the scar.
As TNT noted, the ants are only in the dead wood, creating their nest.
As far as helping the tree, make sure it doesn't get drought stressed, Although I'm not too concerned with a scar and cavity that size, if you like the tree, I would recommend that you keep it pruned to a relatively small size. Use proper pruning techniques.