They don't differ significantly from atheists when it comes down to rejection of all the others.
I think they do.
Christains don't believe in Thor because he's a False God, not to be confused with the real one. When they hear 'God,' there's a specific deity they think of first. That space is taken and they distinguish between their god (s) and the false ones.
...
I think this is an important point. The subtext of Lion's posts on the subject - including his claim that he would rather people be Muslims than Atheists - gives weight to this idea, and even the Bible seems to support it; To many Christians, all the Gods are real, and theirs is the top God, the best one, and the only one worthy of worship. The others are 'false' or 'lesser' in many ways, and worshiping them is going to upset the big God, so you shouldn't do it; But not because these other Gods don't exist.
Christianity has never really shaken off its polytheistic origins, and so many Christians (and I suspect Lion is amongst them) use the word 'disbelieve' in a VERY different way to the way atheists use it. In short, all the crazy and irrational things Lion suggests about Atheistic attitudes to God (that they are angry at God, or that they should be concerned with God's reactions to their atheism) make sense if by 'I do not believe in Thor' you mean "Thor is real but I am not in his supporters club because MY God is better than Thor".
In Lion's world, everyone has an opinion on which is the correct God, just as everyone has an opinion on which football team will win the cup. You might support the wrong team, or you might even not support any team at all; But nobody seriously believes that
no team will win the cup this year. They might tell you that they don't know, or don't care who wins. There are plenty of people who hate the fact that football takes up so much airtime, and dominates conversation, and they might go so far as to say "Don't even mention the football in my presence", but even those people don't think that football doesn't exist, and even if they don't have an opinion on who will win the cup, they still know that a competition of some kind will be won at some point.
The idea that football doesn't exist AT ALL is one that even the most anti-football person does not hold; And in Lion's worldview, atheists are like those anti-football people who want nothing to do with the sport they hate - and he wants us to get the benefit of enjoying the game itself, even if he can't get us all supporting the winning team (which he is sure will be his own).
Of course, the problem for this worldview is that at it's heart it is wrong - the sport he invests so much time in
really doesn't exist. It's not football at all - it's Quidditch; A purely fictional sport with no games, no players, no winners or losers, and no cup. He is vehement in his unwavering support for Griffindor to win the Quidditch, and cannot grasp that people who say "Quidditch doesn't exist' are not just angry at the fact that Griffindor keep winning every year, or bitter supporters of Hufflepuff, or even uninterested in sports - they really don't think that there is an actual sport being played at all. Nobody will win the cup; Cheering for the wrong team is not better than cheering for no team at all. There are match previews and supporters clubs and team paraphernalia, but no actual games. Ever. Because the game can only be played using magic, and magic
isn't real.
A supporter of Ravenclaw does not have the same reason to reject the claim that Slytherin will win the cup as a person who knows Quidditch doesn't exist. Similarly, Christians do not have the same basis for rejecting Islam as Atheists have for rejecting both.
One of the reasons why atheists find Lion's (and other Christian's) arguments so pathetic is that they are based on the incorrect assumption that atheists believe that Gods exist, but have chosen to reject them all (just as Christians reject all non-Christian Gods). The fact that atheists don't believe any gods exist at all is incomprehensible and inconceivable to these theists, and never enters their reasoning.