This is just plain poor logic. I think most, if not all, religions fall under the category of philosophy. The difference is that they’re based on scripture or teachings from a prophet and will almost always address certain areas (why we’re here, what happens when we die, moral behavior, etc) rather than some less specific assertion, observation, or opinion that may or may not address anything. So, sure, I think it’s fair to say Christianity is a philosophy, but so is Judaism, Islam, Buddhism, whatever. The bad logic here is in saying because it’s one thing, it’s not also another. It’s sort of like saying my dog isn’t a canine because he’s a mammal.
Ultimately, this is just a self-serving assertion because he wants the government to promote Christian ideology. I’ve also heard plenty of other bizarre explanations for why Christianity isn’t a religion too. For instance, I remember being told that it was actually a relationship rather than a religion, because religion doesn’t address the personal aspect of God and you just have to follow a bunch of rules to win his favor, but even as a by-the-book Episcopalian at the time, I thought that was dumb, since there’s plenty of rules we were supposed to follow. They just wanted to draw some separation between Christianity and other religions.
The thing is, as mentioned upthread, this can just as easily backfire and ultimately probably would. Sure, Christianity wouldn’t go anywhere now, but what happens if it starts to lose favor? What happens if some important change occurs and it gets squashed? Like, say, Christ comes back, but now the government is controlled by satan (we’re in the thousand years of darkness now, right? ;)) and has the power to oppress the people who want to follow him?
So, yeah, not only is it just plain bad logic, but it’s self-serving purpose would end up causing as much or more harm than good.