So after struggling for a couple of hours, I figured it out. The main issue was that I did not have access to the source code of the demo, but noticed using rostopic that turtle_tf2_message_broadcaster.py was publishing the PointStamped message with "/world" frame. I had to remove the command that runs that script from the launch file of the demo, implemented my own turtle_tf2_message_broadcaster.py without the '/' in the "world" frame, and then everything worked beautifully.