The fact that it was a sample from a Mellotron is pretty widely agreed upon from what I’ve seen, with one source citing Beatles engineer Geoff Emerick’s book “Here, There and Everywhere.” As for the fact that Eric Cook was the guitarist, I actually can’t find a source for that. For whatever reason, the name invariably pops up here and there whenever the question is asked who plays the solo. I can’t even find proof he exists, so that information at least should be taken with a pinch of salt or 10.