Bauckham's source theory part 2

I can answer the question I had last post. A few pages further Bauckham does acknowledge that other traditions attached themselves onto eyewitness traditions but apparently does not (yet) concede anonymous ones (but I guess I'll have to keep reading). From page 131,

These three Gospels [Mk, Lk, Jn] all use the literary device of the inclusio of eyewitness testimony in order to indicate the main eyewitness source of their story. This does not, of course, exclude the appropriation also of material from other witnesses, and we shall see that these Gospels also do that.