Just the metal in the two engines is more than enough to compensate for the wooden spars and skin (which does reflect some radar). I also can't see what in its shape reduces radar signature. Sounds like a typical urban legend.
With the old radars, the difficulty to track it could be attributed to it flying in small formations and not in huge gaggles. If it flew low then yes it could be "invisible" and hard to track.
It also depends what "tracking" means. The way the air-war was controlled was with boards, where the markers were updated from time to time by the hand of people in contact with the radar operators. Saying that this is a slow and cumbersome is an understatement. The Mossies cruise speed was almost twice that of the bombers making the prediction of their current location more difficult. In order to intercept something which is as fast as the interceptors, the planes have to be directed with precision to cut the way of the target. If you fail to predict the correct interception point, the fighters will either never find the mossies (small formations and planes are harder to spot visually from a distance), or find themselves far behind in a long chase.
