I’d replace the post.

The foam might work for a hot minute but I’m not even convinced of that. I’d think the stress from the weight of the gate will cause it to fail rather quickly.

If the hollow is up in an area where the hinges attach you might split the post with the foam expanding as it cures.

I believe most of the time he never took product out of the store. He just grabbed a door and went straight to returns with it. Only if they denied the return would he then leave with it.