has an incredible speed in his kicking and footwork. Is it Wing Chun?

I think it is.

I was told that Bruce Lee has learnt the "Searching the Bridge", but not "Thrusting Finger". Nevertheless the central idea of "thrusting finger" is the building up of speed of offencing. Particularly the thrusting of the entire body with the direct help of footwork.

Actually, the importance of the thrusting footwork is that it enhances the power of thrusting finger.

Think of the butterfly knife as the thrusting finger, the magic is from the footwork.