To be honest I know very little. I didn't realise that Tiber Septim was more than one person, and was a little shocked to read that his actions involving the Numidium were less than saintly (killing Arctus and covering it up). I'm also unsure as to what version of events is the more accurate, Arcturian Heresy or Rise of Tiber Septim.
Ok so lets see if I'm on the right track now: The flea/wolf qoute is from a book called "et'Ada, Eight Aedra, Eat the Dreamer". In this book the author is having a pop at the belief in the Arcturian Heresey. The "holy hermit of Anvil" is having a pop at this same author (and at people who share this belief) for not believing the Arcturian Heresey. Close?
It's really starting to confuse me now. The book is implying that Zurin Arctus and Tiber Septim are the same person and that the Underking is the enemy.
I think understanding the context of Et'd Ada, Eight Daedra would be helpful. This is basically a magical message recovered from a Moth Priest who has just zero-summed. He reached reverse enlightenment and realized that because one plus negative one equals zero, he does not exist. Poof. Evaporation. This is the sort of thing that quote is talking about. Keep dreaming and refuse to see the truth, or you'll disappear. And don't listen to the musical translation of that document, or you'll end up the same way.
Just why the prophet doesn't like this idea, I can't really be sure. It's technically true, so maybe he just finds the attitude distasteful. It's rather in the same vein as saying "What's the point of doing anything, life is pointless because you're going to die in the end." It is true that you're going to die, and it is not possible to refute this with concrete reasonining and absolute terms, but most everyone would object to the message.
Et'Ada, Eight Daedra also can be seen as diminishing Talos by pointing out that he is just a play-actor mimicking the patterns of Akatosh and Lorkhan, but the Emperor knew damn well what he was doing, or he wasn't so clever after all, so that doesn't make much sense. It may just be that the Prophet is calling out against the apathy of the common people in addition to the apathy brought on by musing over the realities of Nirn's metaphysics. Shut up, honor your ancestors (they got it right) and save the world.
(I didn't realize the origin of the quote until you pointed it out, hehe.)
And as far as Arcturian Heresy vs Orthodoxy (Rise of Tiber Septim is a fan article and I'm not sure which way it's bias lies) there is no correct version of the story. The Heresy diminishes Zurin Arctus and calls his persuasive story a cover-up. He is the Underking but not the first. It introduces the role of Wulfharth, the reincarnating Nordic king and avatar of Shor, and suggests that he was the main player. It is certain at this point that his heart powered the Mantella, since that's the only way it would be a substitute to the Heart of Lorkhan. Howeve, the other details of the story are a Battle of Red Mountain sort of deal. We can't know if Daggerfall's Underking was undead Zurin, Zurin possessed by Wulfharth, or some mix of the two. The man who became Emperor was Hjalti Early-Beard, a Nord or Breton from High Rock, Cuhlecain's general who may or may not have had the Thu-um. He was a soldier and a schemer.