How is Rviz able to get the /odom and /base_link transform from playing rosbag data?

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I have a rosbag file named husky_navigation.bag. The output of rosbag info husky_navigation.bag produces the following information-

path:        husky_navigation.bag
version:     2.0
duration:    60.0s
start:       Feb 08 2018 22:04:11.00 (1518107651.00)
end:         Feb 08 2018 22:05:10.00 (1518107711.00)
size:        294.7 MB
messages:    3563
compression: none [301/301 chunks]
types:       nav_msgs/Odometry       [cd5e73d190d741a2f92e81eda573aca7]
             sensor_msgs/Imu         [6a62c6daae103f4ff57a132d6f95cec2]
             sensor_msgs/JointState  [3066dcd76a6cfaef579bd0f34173e9fd]
             sensor_msgs/PointCloud2 [1158d486dd51d683ce2f1be655c3c181]
topics:      /husky_velocity_controller/odom       600 msgs    : nav_msgs/Odometry      
             /imu/data                            1190 msgs    : sensor_msgs/Imu        
             /joint_states                        1172 msgs    : sensor_msgs/JointState 
             /velodyne/assembled_cloud_filtered    601 msgs    : sensor_msgs/PointCloud2

I am also running an ekf_localization_node provided in the robot_localization package.

When I add 'TF' in Rviz, I see two frames - 'odom' and 'base_link'.

How is Rviz able to get these these two frames even when I haven't yet launched the robot_state_publisher node? I read the definition of the 'robot_state_publisher' and it says this - 'This package allows you to publish the state of a robot to tf.'