I think the underlying conceit of even writing a sequel to this is terrible. Some things don't need a sequel way too many years after the fact for it to make any sense. See: A Christmas Story, Slap Shot, American Psycho.
Perhaps Alan Moore could pull it off by pulling this all into League of Extraordinary Gentlemen, but those are more homage than sequel
A Christmas Carol wasn't a story with unexplored corners waiting for some intrepid soul to venture into. It shared the tale of a soul plagued with the vices of greed and pride seeing the error of his ways and repenting of his evil.
If someone was going to try and continue the story, then thematically it would mesh if you carried the same exploration into other vices. Instead of greed, it could be someone consumed by gluttony or anger, and Tiny Tim could help them learn some self-control and empathy. Given that Fat has neither of those virtues, he clearly cannot write such a book.
Nothing that Fat has hinted about in his book suggests that it bears any connection whatsoever to the original book, save for the names of a couple of characters, and the setting. Of course, given how Fat portrayed an alien space station as substantively identical to the modern American Midwest, I doubt we're going to get much of the richness of 19th century London... And we certainly won't be getting any characters that are recognizable from the Dicken's books. Tiny Tim was such a pure-hearted boy that he was able to extend love to a man Dickens described as, "...a squeezing, wrenching, grasping, scraping, clutching, covetous, old sinner! Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self contained, and solitary as an oyster." And Tim's generosity of spirit gave Scrooge the inspiration and hope he needed for the change of heart that saved his soul and made him a new man.
And Fat wants to take this paragon of virtue and turn him into a serial killer, hell-bent on revenge.
I cannot express the depths of my contempt for the lowliness of soul and depravity of spirit contained in the corpulent, porcine form of Patrick S. Tomlinson. That man's feeling for his fellow humans is so constrained that it cannot reach beyond the limits of his own skull. He literally cannot write a character that isn't himself, because he cannot fathom that creatures other than himself exist. Every virtue that he sees in someone else is one that he has pretended to himself, every vice he perceives in those he hates is one he gleefully exercises. There isn't an ounce of genuine compassion or empathy for another person in his soul, so the idea of him taking the embodiment of charitable love that is Tiny Tim and defacing him by turning him into the flesh-glove for all the wretchedness that is Pat Tomlinson is utterly repellant to all wholesome sensibilities.
tl;dr: Fat has so much awfulness of character that it couldn't be contained within a single human form, and therefore it made him fat. Not just fat, but morbidly obese. And his writing is an abomination before God.