Bidirectional path tracing is one way of sampling paths (actually it combines many techniques for sampling paths and weights those using something called Multiple Importance Sampling). It's not the only way of doing it. Disney most likely uses path tracing with next event estimation. This means that they start a path as explained in the video and they end a path by sampling a point on a light and then connecting the start of the path with the point to form a full path. This is one the techniques used by bidirectional path tracing (bdpt), this technique uses n vertices on the camera path and 1 vertex on the light path, but bdpt also uses techniques with s vertices on the camera path and t vertices on the light path. This means that there are multiple ways to sample the same path, so these techniques need to be weighted using something called Multiple Importance Sampling.