I think you're missing an extremely important detail: Muslims do not consider Jesus to be God, while the entirety of the Christian faith is based on the notion that he IS. So a muslim (or any religious person who denies that divinity) writing about the "real" Jesus is going to be a very big deal and make people stop and question the author on his motives. Especially considering that massive disagreement is at the hearts if hundreds of years worth of aggression. And the Moses comparison doesn't work because the religions don't disagree on the nature of Moses, nor do the religions worship him as their god.
I haven't read the book yet, so I can't comment on what it says, but I think it's safe to say that the reason anyone would have an issue with a Muslim writing a book about Jesus has NOTHING to do with the author being a Muslim in particular, but it's the simple fact that he is of a faith that believes Jesus was far less than what Christians believe he was, and they would naturally take offense to him using his authority as a scholar and author to try and push his belief of Jesus as nothing more than a simple man or prophet which is a very direct attack on an entire religion's belief (whether the author intends it to be such or not). THAT's why people are saying "hold up, what's this now?"
Couldn't we argue then that his faith would contribute to give us a proper and objective understanding of who historical Jesus was? Are people angry that it's a Muslim writing it or is it because they don't want to know anything other than what's in the bible?
Where did I say anything remotely like that? My point is not that people can't write about other religions; my point is that they can, they just need to understand that people of that religion are going to want to know what the authors intentions are, especially if the topic could concern denying a fundamental aspect of that religion. It's not bad for a Muslim to write about Jesus, and it's not bad for Christians to want to understand his motivations, and the same is true the other way around.


