Errata: the mouse input code in the video needs one adjustment.
In the video, we multiply the mouse input by delta: _camera_input_direction.y * delta. This is not necessary as mouse motion input is already time-dependent: event.screen_relative already tells you how much the mouse moved since the last frame.
It's not a major problem as the delta value in _physics_process() does not change, regardless of the actual render frame rate. But it's still worth knowing that if you move things based on mouse input in the _process() function, multiplying by delta will make the code actually framerate dependent.
Remove the multiplication by delta like this:
func _physics_process(delta: float) -> void:
_camera_pivot.rotation.x += _camera_input_direction.y * delta
_camera_pivot.rotation.x += _camera_input_direction.y
_camera_pivot.rotation.x = clamp(_camera_pivot.rotation.x, tilt_lower_limit, tilt_upper_limit)
_camera_pivot.rotation.y -= _camera_input_direction.x * delta
_camera_pivot.rotation.y -= _camera_input_direction.x
Because we do not multiply by delta anymore, the motion will be much stronger. Lower mouse_sensitivity from 0.25 to a much lower value, around 0.005, to keep a similar camera rotation speed.
I've adjusted the code references below to reflect the changes.