Reference: @ about 4:05 in the video, when he said he had to disable and renable the “Tilemap Collider 2D” component to make the collider update once he made the custom physics shape for the ladder sprite.
The problem: After disabling and renabling the Tilemap Collider 2D component, the collider shape still did not update.
The fix: Click on your “Climbing Tilemap” in the Hierarchy, then in the Inspector on your “Tilemap Collider 2D” component, click the “…” in the top right and click “Reset”. This should refresh the Unity cache on this component, and visually you will see the collider update.