John 1:1-18 added on later?

This paper claims that at least 8 scholars according to Brown “agreed to omitting [verses] 6-8, 15.” Brown suggests that the “evidence is inconclusive” for the rest of the prologue.
Ehrman also has written that he doesn’t think 1-18 was written by the same author as the rest of the gospel. He suggests that John is a composite of 6 or so narratives edited together, most likely not by the same author. He calls the epilogue into question as well.
https://ehrmanblog.org/the-christ-poem-in-john-for-members/
And so it is widely thought that the author of the Fourth Gospel appended these verses as a Prologue, possibly after the rest of the book was written. It is widely thought, in fact, that the Gospel went through multiple editions, so that 1:1-18 is a later addition and the final chapter, ch. 21, is an even later addition.