He was trying to connect with those in the church. He is correct. If they, like him, have been adopted into the family of God, that makes them Brothers and Sisters. Not a big deal, just a statement of fact.
No more controversial than saying you know what? We have the same mother so we are brothers/sisters. Just the facts.
The public square needs more acknowledgments of our need for God, not fewer. I really have no patience with the notion that Americans need to be protected from references to God, including from people who aren’t paid to do so.