A 1178 CP Kabutops fights against a 1401 Golem in the video (0:45 - 1:15). This page shows the details of those Pokemon, the video, and the description.
The attacker, Kabutops, uses Mud Shot. The defender, Golem, is weak to the ground-type fast move. Even though the CP of Kabutops is lower than that of the Golem, Kabutops has an advantage over the defender for the relationship of types.
To win a gym battle in Pokemon Go, type effectiveness is considered as more important than other factors, such as CP and stats.
2.
The defende uses Rock Throw, a rock-type fast move.
3.
Kabutops shoots Mud Shot continuously and rapidly although the required energy for special attack is charged.
4.
Golem uses Earthquake, a ground-type charge move. However, the defender is defeated before its charge move hits the Kabutops.